Multi-layer optical recording media and system for recording and reproducing information data

ABSTRACT

Disclosed is a novel multi-layered optical recording media and system for recording and reproducing information data. The multi-layered topical recording media has M information storage decks, and each information storage deck has N information storage layers, and each information storage layer has a pair of information storage structures. Each paired information storage structure has a characteristic wavelength and polarization state, and from which recorded information can be read by a laser beam having similar wavelength and polarization-state characteristics. In the illustrative embodiment, the multi-layered optical recording media of the present invention has MxNx2 information storage layers which can be read using only N laser lines (i.e. spectral components), thereby providing a 2M-fold increase in information storage capacity over prior art systems. The information storage and retrieval system of the present invention is completely backward compatible to allow for the reading of conventional CD-ROM devices.

RELATED CASES

This Application is a Continuation of Application Ser. No. 08/539,279entitled “Method And Apparatus for Storing And Retrieving InformationUsing Optical Data Storage Media”, by the inventors Bunsen Fan and SadegM. Faris, filed Oct. 4, 1995 and now is U.S. Pat. No. 5,838,653.

BACKGROUND OF INVENTION

1. Field of the Invention

The present invention relates to an information storage system employinga novel optical-based information storage media having multipleinformation storage layers each of which have a characteristicwavelength and polarization state, and from which recorded informationcan be read by a laser beam having similar wavelength andpolarization-state characteristics in order to provide a significantimprovement in information storage capacity.

2. Brief Description of Prior Art

In the contemporary period, there is a great need for high densityinformation storage media. For decades, magnetic disc and tape have beenthe dominant information storage media for recording both analog anddigital information. In more recent times, the trend has changed tooptical storage media because of its higher information storagecapacity. In principle, each optical storage media exploits one or morecharacteristics of light and its interaction with matter in order tostore and access information.

In conventional optical CD-ROM technology, digital information in theform of a logical “0” is recorded as a microscopic protrusion formed onthe reflective surface of the compact disc along a continuous spiraltrack, whereas a logical “1” is recorded as a microscopic section ofplanar surface area the size of about the cross-sectional diameter ofthe focused laser beam used during information reading operations. Whenan incident laser beam is reflected off such microscopic protrusionsduring information reading operations, a reduction in intensity of thereflected laser beam is detected by the photodetector of the CD-ROMdrive unit and is converted into a logical signal. In conventionalCD-ROM technology, the primary limitation on information storagecapacity is determined by protrusion size. Consequently, great effortsare being undertaken to make inexpensive laser diodes that produce laserbeams with shorter wavelengths for use in detecting smaller-sizedprotrusions during information reading operations.

Recently, there have been attempts to increase the information storagecapacity of conventional CD-ROM devices. One technique in particular,which has received popular attention, is disclosed in U.S. Pat. No.5,381,401. In essence, this technique involves stacking up multipleinformation layers of optically, substantially transmissive material,each realized by an regular CD disc having a thin aluminum film coatingwith a light reflectance of about 4%. By adjusting the depth of focus ofthe laser beam used during information reading operations, it ispossible to read information from a selected information storage disc.However, a major drawback with this prior art technique is that only arelatively small number of information layers can be stacked up beforesevere attenuation of the reflected laser beam occurs, thereby resultingin an unacceptable signal-to-noise ratio and thus system performance.

An altogether different approach to optical mass storage has been taughtin recently issued U.S. Pat. No. 5,353,247 to Faris, co-applicant of thepresent application. Rather than using semi-reflective coatings, astaught in U.S. Pat. No. 5,381,401, U.S. Pat. No. 5,353,247 teaches theuse of the wavelength-selective property of the Cholesteric LiquidCrystal (CLC) material, and discloses an optical storage mediacomprising multiple information storage layers made of CLC material.However, a major drawback with this prior art system is that N laserlines (i.e. spectral components) are required to read N number ofCLC-based information storage layers. How this shortcoming might beovercome is neither disclosed, taught or suggested by the prior art.

Thus there presently is a great need in the art for an improved methodand apparatus for storing and retrieving information in a manner whichachieves a substantial improvement in information storage capacity overprior art systems and methodologies.

OBJECTS AND SUMMARY OF THE INVENTION

Accordingly, a primary object of the present invention is to provide anovel optical information storage media which overcomes the shortcomingsand drawbacks of prior art apparatus and methodologies.

Another object of the present invention is to provide novel informationstorage media which exploits both the wavelength-dependent andpolarization-dependent reflective properties of particular opticalmaterials.

Another object of the present invention is to provide a novel opticalinformation storage structure which exploits both thewavelength-dependent and polarization-dependent reflective properties ofCLC material.

Another object of the present invention is to provide a novel opticalinformation storage structure comprising a pair of information storagelayers, each of which exploit both the wavelength-dependent andpolarization-dependent reflective properties of CLC material ormultilayer dielectric thin-films so that digital information storedtherein can be read in either a sequential or parallel manner.

Another object of the present invention is to provide a novel opticalinformation storage system which employs an optical storage media havinga single information storage deck with 2xN CLC-based information storagelayers which can be read using only N laser lines (i.e. spectralcomponents).

Another object of the present invention is to provide a novel opticalinformation storage system which employs an optical storage mediumhaving M information storage decks, wherein each storage deck has 2NCLC-based information storage layers, and the MxNx2 information storagelayers can be read using only N laser lines (i.e. spectral components).

Another object of the present invention is to provide such an opticalinformation storage system, in which an optical pick-up is employed thatcontrols the polarization state of the laser beam as it is incident uponthe optical storage medium during information reading operations.

Another object of the present invention is to provide such an opticalinformation storage system, in which an optical pick-up is employed thatcontrols the polarization state of the laser beam after it reflects offthe optical storage medium during information reading operations.

Another object of the present invention is to provide a novel opticalinformation storage media which exploits both the wavelength-dependentand polarization-dependent reflective properties of CLC material, andwhich can be realized in the form of a compact disc, tape, or card.

Another object of the present invention is to provide a novel method ofmanufacturing multilayered optical-based storage media having wavelengthand polarization-state dependent characteristics.

Another object of the present invention is to provide a novel method ofmanufacturing an optical information storage structure which exploitsboth the wavelength-dependent and polarization-dependent reflectiveproperties of CLC material.

Another object of the present invention is to provide a novel method ofmanufacturing a multi-layer information storage structure which exploitsboth the wavelength-dependent and polarization-dependent reflectiveproperties of multilayer dielectric thin-films.

Another object of the present invention is to provide a novel opticalpick-up for reading digital information recorded in a multi-layeredinformation storage structure which exploits both thewavelength-dependent and polarization-dependent reflective properties ofCLC material.

Another object of the present invention is to provide a novel opticalpick-up for reading digital information recorded in a multi-layeredinformation storage structure which exploits both thewavelength-dependent and polarization-dependent reflective properties ofmultilayer dielectric thin-films.

Another object of the present invention is to provide a novel opticalpick-up for simultaneously reading digital information stored in theinformation recording tracks of a paired multi-layered informationstorage structure which exploits both the wavelength-dependent andpolarization-dependent reflective properties of either CLC material ormultilayer dielectic thin-films.

Another object of the present invention is to provide a novelinformation storage disc drive unit capable of reading CLC-basedinformation storage discs of the present invention, as well asconventional CD-ROMs, thus providing an easy and inexpensive way toupgrade the information storage and reading capabilities of a computersystem, without the need to discard conventional CD-ROMs in thepossession of its user.

Another object of the present invention is to provide a novel method andapparatus for precisely aligning the information recording tracks formedin the information storage layers of a paired information storagestructure.

These and other objects of the present application will become apparenthereinafter and in the Claims to Invention.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the Objects of the PresentInvention, the following Detailed Description of the IllustrativeEmbodiments should be read in conjunction with accompanying Drawings, inwhich:

FIG. 1 is a schematic representation of the generalized embodiment ofthe information storage and retrieval system of the present invention;

FIGS. 1A, 1B, and 1C set forth a set of schematic representations of (i)a constant intensity laser beam (i.e. continuous wave laser beam)directed incident onto an information storage structure of the presentinvention, (ii) the reflected intensity of the laser beam afterreflecting off the information storage structure, and (iii) the logicalinformation signal recovered from the reflected laser beam afterdetection and processing by the system of the present invention;

FIGS. 1D, 1E and 1F set forth is a set of schematic representations of(i) a high-frequency modulated laser beam (e.g. three times theinformation recording rate) directed incident onto an informationstorage structure of the present invention, (ii) the reflected intensityof the laser beam after reflecting off the information storagestructure, and (iii) the logical information signal recovered from thereflected laser beam detection and processing by the system of thepresent invention;

FIG. 2 is a schematic representation of the address translator of theinformation storage and retrieval system of FIG. 1, illustrating theconversion of logical addresses into physical addresses within theoptical storage media of the present invention;

FIGS. 3A and 3B set forth a perspective view of an information storageand retrieval system of the present invention, particularly adapted foruse with optical storage media of the present invention realized in theform of compact disc;

FIG. 3C is a schematic representation of CD optical disc being rotatedat an angular velocity which is dependent upon the instantaneousposition of the optical reading head of the information reading system;

FIG. 3D is a schematic representation of an information storage andreading system constructed in accordance with the present invention, inwhich dual optical pick-ups are used to simultaneously read differenttracks of information prerecorded on an CLC optical disc of the presentinvention using Zoned Constant Angular Velocity (ZCAV) formatting;

FIGS. 4A, 4B and 4C set forth a perspective view of an informationstorage and retrieval system of the present invention, particularlyadapted for use with optical storage media of the present inventionrealized in the form of a compact card;

FIGS. 5A and 5B set forth a perspective view of an information storageand retrieval system of the present invention, particularly adapted foruse with the optical storage media of the present invention realized inthe form of a compact tape cassette;

FIG. 6 is a schematic representation of the generalized embodiment ofthe optical storage media of the present invention having up to 2Ninformation-storage layers, and comprising M decks, each of whichincludes N paired wavelength-selective and polarization-selectiveinformation storage structures which are distinguishable from eachother, and thus accessible, by the wavelength and polarization state ofthe optical signals used to read information stored therein;

FIG. 7A is a schematic representation of a first illustrative embodimentof the wavelength-selective and polarization-selective informationstorage structure of the present invention which can be used toconstruct the optical storage media hereof, and as shown, comprises (i)a first flat information storage layer which reflects incident lighthaving P₁-polarization state and wavelength λ_(i) wherever a binary bit‘1’ is written and transmits incident light wherever a bit ‘0’ iswritten regardless of wavelength and polarization, and (ii) a secondflat information storage layer which reflects light havingP₂-polarization state and wavelength λ_(i) wherever a binary bit ‘1’ iswritten and transmits incident light whenever a binary bit ‘0’ iswritten regardless of wavelength and polarization;

FIGS. 7B and 7C set forth a schematic representation of the reflectancespectra of the information storage layers of the information storagestructure shown in FIG. 7A;

FIG. 7D is a table setting forth the polarization states of the opticalsignal retrieved (by reflection) from identified information valuestorage locations along each accessed information storage layer in theoptical storage media realized using the paired wavelength-selective andpolarization-selective information storage structure shown in FIG. 7A;

FIG. 8A is a schematic representation of a second illustrativeembodiment of the paired wavelength-selective and polarization-selectiveinformation storage structure of the present invention which can be usedto construct the optical storage media hereof, and as shown, comprises(i) a first planar information storage layer which reflects incidentlight having P₁-polarization state and wavelength λ_(i) whenever abinary bit ‘1’ is written and transmits incident light wherever a binarybit ‘0’ is written regardless of wavelength and polarization, and (ii) asecond flat information storage layer which reflects light havingP₂-polarization state and wavelength λ_(i) wherever a binary bit ‘1’ iswritten and transmits incident light whenever a binary bit ‘0’ iswritten regardless of wavelength and polarization;

FIGS. 8B and 8C set forth a schematic representation of the reflectancespectra of the information storage layers shown in FIG. 8A;

FIG. 8D is a table setting forth the polarization states of the opticalsignal retrieved (by reflection) from identified information storagelocations along each accessed information storage layer in the opticalstorage media realized using the paired wavelength-selective andpolarization-selective information storage structure shown in FIG. 8A;

FIG. 9A is a schematic representation of a third illustrative embodimentof the paired wavelength-selective and polarization-selective storagestructures of the present invention which can be used to construct theoptical storage media hereof, and as shown, comprises (i) a firstundulated information storage layer with a reflective coating thatreflects incident light having P₁-polarization state and wavelengthλ_(i) the same amount whenever a binary bit ‘1’ or a binary bit ‘0’ iswritten but which is detectable at different signal levels by an opticalpick-up due to surface height differences for the binary bit ‘1’ andbinary bit ‘0’, and (ii) a second undulated information storage layerwith a reflective coating that reflects incident light havingP₂-polarization state and wavelength λ_(i) the same amount whenever abinary bit ‘1’ or a binary bit ‘0’ is written but which is detectable atdifferent signal levels by an optical pick-up due to surface heightdifferences for the binary bit ‘1’ and binary bit ‘0’;

FIGS. 9B and 9C set forth a schematic representation of the reflectancespectra of the information storage layers specified in FIG. 9A;

FIG. 9D is a table setting forth the polarization states of the opticalsignal reflected (i.e. retrieved) from identified information storagelocations along each accessed storage layer in the optical storage mediarealized using the paired wavelength-selective andpolarization-selective storage structure shown in FIG. 9A;

FIG. 9E is a schematic representation of a fourth illustrativeembodiment of the paired wavelength-selective and polarization-selectiveinformation storage structure of the present invention which can be usedto construct the optical storage media hereof, and as shown, comprises(i) a first planar, thin-film type information storage layer whichreflects incident light having P₁-polarization state and wavelengthλ_(i) wherever a binary bit ‘1’ is written and transmits incident lightwherever a binary bit ‘0’ is written regardless of wavelength andpolarization, and (ii) a second planar, thin-film type informationstorage layer which reflects light having P₂-polarization state andwavelength λ_(i) wherever a binary bit ‘1’ is written and transmitsincident light wherever a binary bit ‘0’ is written regardless ofwavelength and polarization;

FIGS. 9F and 9G set forth a schematic representation of the reflectancespectra of the thin-film type information storage layers shown in FIG.9E;

FIG. 9H is a table setting forth the polarization states of the opticalsignal retrieved (by reflection) from identified information storagelocations along each accessed information storage layer in the opticalstorage media realized using the paired wavelength-selective andpolarization-selective information storage structure shown in FIG. 9E;

FIG. 10A is a schematic representation of an embodiment of the opticalstorage media of the present invention, shown constructed from two pairsof wavelength-selective and polarization-selective information storagestructures of the present invention, shown above in FIGS. 7A to 9D, andfrom which prerecorded information tracks on different structures can besimultaneously read using a pair of optical signals having differentcharacteristic wavelengths λ₁ and λ₂, respectively, during informationreading operations;

FIGS. 10B, 10C, 10D and 10E set forth a schematic representation of thereflectance spectra of each of the information storage layers shown inFIG. 10A;

FIG. 11A is a schematic representation of a first illustrativeembodiment of the information storage system illustrated in FIG. 1,wherein information prerecorded in each pair of wavelength-selective andpolarization-selective information storage structures in eachinformation storage deck of the 2-deck system, can be read duringinformation reading operations using a single focused laser beam havinga characteristic wavelength λ_(I);

FIG. 11B is a schematic diagram of the spherical abberation compensatorwithin the optical reading pick-up of the present invention shown in afirst configuration;

FIG. 11C is a schematic diagram of the spherical abberation compensatorwithin the optical reading pick-up of the present invention shown in asecond configuration;

FIGS. 12A and 12B set forth a schematic representation of a secondillustrative embodiment of the information storage system illustrated inFIG. 1, wherein information prerecorded in each i-thwavelength-selective and polarization-selective information storagestructure in each of the M information storage decks of the system, canbe read during information reading operations using (i) a single focusedlaser beam having a characteristic wavelength λ_(i), (ii) a mechanismfor moving the focused laser beam up and down along a particularinformation storage deck, and (iii) a mechanism for fine-focusing theincident laser beam onto particular wavelength-selective andpolarization-selective information storage structure within eachinformation storage deck and tracking the laser beam reflectedtherefrom;

FIG. 13A is a schematic representation of a fifth illustrativeembodiment of the paired wavelength-selective and polarization-selectivestorage structures of the present invention which is based on thewavelength-selective and circular polarization-selective reflectanceproperties of CLC material, which can be used to construct the opticalstorage media hereof, and as shown, comprises (i) a first planarinformation storage layer which reflects incident light having aright-handed circularly polarized (RHCP) state and wavelength λ_(i)wherever a binary bit ‘1’ is written and transmits incident lightwherever a binary bit ‘0’ is written regardless of polarization andwavelength, and (ii) a second planar information storage layer whichreflects incident light having a left-hand circularly polarized (LHCP)state and wavelength λ_(i) wherever a binary bit ‘1’ is written andtransmits incident light wherever a binary bit ‘0’ is written regardlessof polarization and wavelength;

FIGS. 13B and 13C set forth a schematic representation of thereflectance spectra of the information storage layers specified in FIG.13A;

FIG. 13D is a table setting forth the polarization states of the opticalsignal retrieved from identified information value storage locationsalong each accessed information storage layer in the pairedwavelength-selective and polarization-selective storage structures shownin FIG. 13A;

FIG. 14 is a schematic representation of second illustrative embodimentof the information storage system illustrated in FIG. 1, whereininformation prerecorded in each one of the pair of wavelength-selectiveand polarization-selective information storage structures of each i-thinformation storage layer in the m-th information storage deck of thesystem, can be read by using a single focused laser beam having acharacteristic wavelength λ_(i) and corresponding polarization state(i.e. LHCP or RHCP), and a mechanism for moving the focused laser beamup or down to a particular information storage deck of the system duringinformation accessing operations;

FIGS. 15A to 15D collectively show the sequence of steps carried out bythe method of manufacturing multi-decked information storage structuresrealized using CLC material;

FIG. 16 is a perspective schematic representation of a fifthillustrative embodiment of the pair of wavelength-selective andpolarization-selective storage structures according to the presentinvention which is based on multilayer dielectric thin film material,can be used to construct the optical storage media hereof, and as shown,comprises (i) a first planar information storage layer realized as afirst dielectric thin-film layer which reflects incident light having afirst linearly polarized state P1 and wavelength λ_(i) wherever a binarybit ‘1’ is written and transmits incident light wherever a binary bit‘0’ is written regardless of polarization and wavelength, and (ii) asecond planar information storage layer realized as a second dielectricthin-film layer which reflects incident light having a second linearlypolarized state P₂ (orthogonal to P₁) and wavelength λ_(i) wherever abinary bit ‘1’ is written and transmits incident light wherever a binarybit ‘0’ is written regardless of polarization and wavelength;

FIG. 17A is a schematic diagram of a type-1 optical pick-up whichcontrols the polarization state of the spectral components of theincident laser beam focused onto the optical storage media duringinformation reading operations;

FIG. 17B is a schematic diagram of an optical subsystem that can be usedwith the optical pick-up of FIG. 17A, to control the polarization stateof the spectral components of the incident laser beam used duringinformation reading operations;

FIG. 17C is a schematic diagram of another type of optical pick-up inwhich only spectral components transmitted through the optical storagemedia hereof are detected during information reading operations;

FIG. 17D is a schematic diagram of an optical subsystem that can be usedwith the optical pick-up of FIG. 17C, to control the polarization stateof the spectral components of the incident laser beam used duringinformation reading operations;

FIG. 17E is a schematic diagram of an optical subsystem that can be usedwith the subsystem of FIG. 17A, to detect the reflected spectralcomponents along isolated optical channels;

FIG. 17F is a schematic diagram of the aperture of a focusing objectivelens used in an optical pick-up of the present invention, in which theincident and reflected laser beams are not allowed to spatially overlap;

FIG. 17G is a schematic diagram of the aperture of the focusingobjective lens used in an optical pick-up of the present invention, inwhich the incident and reflected laser beams are allowed to spatiallyoverlap using retro-reflective principles;

FIG. 17H is a schematic diagram of an optical subsystem that can be usedwith the subsystem of FIG. 17C, to detect along isolated opticalchannels, both the reflected spectral components for feedback signalgeneration used in autofocusing and tracking, as well as (ii)transmitted spectral components for recovering the information patternrecorded along an information storage track;

FIG. 17I is a schematic diagram of different information storage tracksrecorded on an optical storage disc of the present invention, when usingZoned Constant Angular Velocity formatting;

FIG. 18 is a schematic diagram of an optical pick-up which permits theuse of a light focusing objective lens having a larger aperture, andwhich uses a knife-edge technique for laser beam auto-focusing;

FIGS. 18A, 18B and 18C set forth a schematic representation showingdifferent light intensity distributions being focused onto the duallight detector of the system of FIG. 18, for the three different casesof laser beam focusing under servo-control therein;

FIG. 19 is a schematic diagram of an optical pick-up which permits theuse of a light focusing objective lens having a larger aperture, byusing a Faraday optical isolator;

FIGS. 19A, 19B and 19C set forth a schematic representation showingdifferent light intensity distributions being focused onto the duallight detector of the system of FIG. 19, for the three different casesof laser beam focusing under servo-control therein;

FIG. 19D is a schematic representation of an exemplary section of aninformation storage layer of the present invention, showing aninformation track along which an incident laser beam is undergoingservo-control during auto-tracking in order to prevent it from beingmoved accidentally onto adjacent information storage tracks duringinformation reading operations;

FIGS. 19E, 19F and 19G set forth a schematic representation showingdifferent light intensity distributions being focused onto the dualphotodetector of the optical pick-up of FIG. 19, for the three differentcases of laser beam tracking under servo-control therein.

FIG. 20A is schematic diagram of a type-2 optical pick-up which detectsthe polarization state of laser beam after reflecting off theinformation storage layers of the optical storage medium duringinformation reading operations;

FIG. 21 is a schematic diagram of a non-retroreflective opticalsubsystem for simultaneous reading of aligned information stored in apaired polarization-selective and wavelength-selective informationstorage layers;

FIG. 22 is a schematic representation of the information storage andretrieval system of the present invention, in which an automaticdisc-type recognizing mechanism is employed to provide backwardcompatibility;

FIG. 23 is a schematic representation illustrating the logicaloperations carried out by the automatic disc-type recognizing mechanismemployed in the system of FIG. 22;

FIGS. 24A to 24F show the process steps for making master molds formass-manufacturing CLC-based information storage discs;

FIGS. 25A to 25F collectively show the process steps for makingdouble-sided CLC-based disks using lamination techniques;

FIGS. 26A to 26D collectively show the process steps for makingdouble-sided CLC-based disks using a single stamping operation;

FIG. 27A is a schematic diagram of a double-sided CLC-based informationstorage disc of the present invention, showing the physical location ofalignment marks thereon;

FIG. 27B is a schematic diagram of the alignment marks formed on thefirst and second sides of the CLC-based information storage disk;

FIG. 27C is a schematic diagram of a first embodiment of an opticalsubsystem for precisely aligning the alignment marks on the first andsecond sides of the double-sided CLC discs;

FIG. 27D is a schematic diagram showing the electrical signals producedby the subsystem of FIG. 27C during the alignment process;

FIG. 27E is a schematic diagram of a second embodiment of the opticalsubsystem hereof, for aligning the alignment marks on the first andsecond sides of double-sided CLC-based information storage discs duringmanufacturing operations carried out using the method of FIG. 25;

FIG. 27F is a schematic diagram showing the electrical signals producedby the subsystem of FIG. 27E during the alignment of the informationvalue patterns on the first and second information storage structures ofthe CLC-based information storage disc;

FIG. 28 is a schematic diagram illustrating the steps of a process formaking an aligned CLC-based information storage disk using a singlestamping operation;

FIG. 29 is a schematic diagram illustrating the steps of a process formaking an aligned double-sided CLC-based information storage disc usingan embossing operation;

FIGS. 30A and 30B set forth a schematic diagram for making an embossingdrum for use in manufacturing CLC-based information storage discs usingthe process of FIG. 29;

FIG. 31 is a schematic diagram of the apparatus and process for aligningthe embossing drums used in the manufacturing method of FIG. 29;

FIG. 32A is a schematic representation of a portion of an informationtrack formed on the optical storage media of the present invention, inwhich the information pattern has been encoded using afrequency-modulated (FM) analog signal, rather than a digital signal;

FIG. 32B is a schematic representation of the digital signal produced byan optical pick-up of the present invention, detecting an incident laserbeam reading the information pattern shown in FIG. 32A;

FIG. 32C is a schematic representation of the demodulated analog signalrecovered from detected digital signal of FIG. 32B; and

FIGS. 33A to 33G collectively show the sequence of steps carried outduring the method of manufacturing a paired information storagestructure of the present invention using artificial chiral films.

DETAILED DESCRIPTION OF THE ILLUSTRATIVE EMBODIMENTS OF THE PRESENTINVENTION

Referring to the figures set forth in the accompanying Drawings, theillustrative embodiments of the present invention will be described indetail hereinbelow. For clarity of exposition, like figures shown in theaccompanying Drawings shall be indicated with like reference numerals.

In FIG. 1, the information storage and retrieval system of the presentinvention is schematically illustrated. As shown, the informationstorage and retrieval system 1 is operably connected to a host processor2 typically associated with a computing system, such as a desktopcomputer, laptop computer, etc. The system may be constructed as astand-alone unit external to the computer-based system, or it may beconstructed so that it is internally incorporated into the housing ofthe computer-based system.

In general, the information storage and retrieval system 1 comprises anumber of functional subcomponents, namely: an optical-based informationstorage device 3 realized in the form of a disc, tape, card or likestructure, constructed from the wavelength and polarization selectiveinformation recording structure of the present invention; a storagecontroller 4 and address translator 5 for controlling system operations;a media access controller 6 for controlling the polarization andwavelength characteristics of the laser light beam used duringinformation reading operations; a plurality of laser sources (e.g. laserdiodes) 7 for producing a plurality of different laser lines (i.e.spectral components) having characteristic wavelengths λ₁, . . . . . . ,λ_(N), respectively, at which peak spectral output occur laser sourcecontrol mechanism 8 for selecting and controlling one or more of theplurality of laser sources in order to produce a composite laser beamwith preselected polarization and wavelength characteristics; a lightpolarizing mechanism 9 for selectively polarizing (e.g. circularly orlinearly polarizing) each of the produced laser lines in response topolarizing-state specifications {P_(j)} produced by the media accesscontroller 6; a laser beam collimating/shaping mechanism 10 forcollimating and shaping (e.g. circularizing) the collimated beam to forma circularized, composite laser beam for use in reading informationprerecorded on the optical storage disc 3 hereof; a first beam steeringmechanism 11 for steering across the optical storage device, theincident composite laser beam having wavelength and polarizationcharacteristics {λ_(i), P_(j)}; depth focusing mechanism 12 for focusingthe depth of the focused composite laser beam according to depth controlinformation Z(m,n); auto-focusing and tracking mechanism 13 forauto-focusing and tracking the composite laser beam as it is beingsteered across the optical storage disc; a servo-control mechanism 14for servo-controlling the auto-focusing and tracking mechanism 13; abeam focusing mechanism 15 for focusing the composite laser beam to thespecified depth in the optical storage media; first moving mechanism 16for moving the beam focusing mechanism 15 in a coordinate directionspecified by the storage system controller; second moving mechanism 17for moving or rotating the optical storage media 3 in a coordinatedirection specified by the storage system controller; a second beamsteering mechanism 18 for steering the composite laser beam along aspecified direction as it is reflected from the optical storage device;wavelength and polarization-state detection mechanism 19 for controllingthe wavelength and polarization characteristics of the reflected laserbeam for subsequent optical signal processing; an optical-electricalsignal conversion mechanism 20 for converting the received analogoptical signal into a corresponding electrical signal; ananalog-to-digital (A/D) signal conversion mechanism 21 for convertingthe analog electrical signal into a corresponding digital signal; and adigital signal processor 22 (shown contained within controller 4) forprocessing the digital signals in order to retrieve the digitalinformation encoded therein, produce digital information representativethereof, and then transfer this digital information to the hostprocessor 2, as shown. In general, each of these functionalsubcomponents may be realized in a number of different ways, dependingon the technology available at the time of implementation. However, forpurposes for illustration, each these subcomponents will be described ingreater detail with respect to the illustrative embodiments of thepresent invention.

In general, the optical storage device 3 indicated in FIG. 1 canrealized as a disc, tape, or card as shown in FIGS. 3A to 5B,respectively. In each embodiment of the present invention, the opticalstorage device is constructed using a novel information recordingstructure of composite (i.e. multi-layer) construction realized in asuitable optical medium. Each information recording layer in themulti-layer structure has both a characteristic wavelength andpolarization state which imparts thereto light reflective propertieswhich, as described in U.S. Pat. No. 5,353,247, are dependent on bothwavelength and polarization-state of the incident light beam. Notably,“characteristic wavelength” is defined as the wavelength of peakreflection of the optical storage layer, whereas the “characteristicpolarization state” is defined as the polarization direction in whichlight at the characteristic wavelength is directed by the opticalstorage layer. At information recording cell locations where a logical“1” has been recorded, the physical effect of such properties is tocause the information recording layer to reflect any incident laser beamcomponent having a characteristic wavelength and polarization statewhich is the same as the characteristic wavelength and polarizationstate of the information recording layer upon which the laser beam isincident. At information recording cell locations where a logical “0”has been recorded, the physical effect of such properties is to causethe information recording layer to transmit any incident laser beamcomponent having a characteristic wavelength and/or a polarization statewhich is different from the characteristic wavelength and a polarizationstate of the information recording layer.

In a first illustrative embodiment of the present invention, thecomposite information recording structure can be constructed so that thecharacteristic wavelengths of its adjacent pair of information recordinglayers are identical, and the polarization states of its pair ofinformation recording layers are orthogonal (i.e. in differentdirections). In an alternative, and perhaps less desirable embodiment ofthe present invention, the composite information recording structure canbe constructed so that the characteristic polarization states of itsadjacent pair of information recording layers are identical, and thecharacteristic wavelengths of its pair of information recording layersare different. As will be described in greater detail hereinafter, thecomposite information recording structure itself can be stacked inmultiple layers up to a layer-depth N, and the stack of N compositeinformation storage structures itself can be stacked in decks up to adeck-depth M. Thus the resulting information structure has a 2M-foldincrease in information storage capacity over prior art informationstorage techniques including those taught in U.S. Pat. No. 5,353,247.

The function of the plurality of laser diodes is to selectively producespectral components having output wavelengths λ_(i), where i=1, 2, . . ., N. While not essential, it is preferred that λ₁<. . . <λ_(i)<. . .<λ_(N). During any information reading operation, any one or more theselaser diodes can be simultaneously selected by way of the laser sourceselection and control mechanism 8, under the control of the media accesscontroller. Preferably, mechanism 8 realized using electronic circuitry.

In general, several types of incident laser beams may be used to readinformation values recorded in the optical media of the presentinvention. As shown in FIGS. 1A, 1B and 1C, the incident laser beamusing during information reading operations may have a constant incidentintensity. Alternatively, the incident laser beam may have an incidentintensity that is modulated at a high frequency (e.g. at three times ofthe information recording rate), as shown in FIGS. 1D, 1E and 1F. Thereare several advantages obtained when using a modulated laser lightsource, namely: the light beam produced therefrom has a relatively shortcoherence length and the deleterious effects of optical feedback arereduced. In practice, there are several ways to produce such modulatedlaser beams. One way is to use a specially designed edge-emitting lightemitting diode (ELED), which has a broad-band spectral output. Anotherway is to use a push-pulsating laser diode, which produces a modulatedlaser beam. The laser beam produced from a self-pulsating laser diodenaturally oscillates at a radio frequency (e.g. 10⁹ Hz) and thus havespectral output wavelengths of 0.78 nanometers, even when driven by a dcexcitation voltage. Such lasers are widely used in the optical pick-upsof conventional CD-ROM drives. Another way to produce a modulated laserbeam is to directly modulate a generated laser beam using a RF signal ina manner well known in the art.

During information reading operations, the intensity property of eachspectral component in the incident laser beam is modulated (i.e.,changed) upon reflecting off an information storage layer havingwavelength and polarization-state characteristics identically matchedwith the spectral components of the incident laser beam. In oneillustrative embodiment of the information storage and retrieval system,the reflected spectral components in the incident laser beam areintensity-modulated by the planar surface characteristics of thereflecting information storage layer. In another illustrativeembodiment, the reflected spectral components in the incident laser beamare intensity-modulated by the stepped (or undulated) surfacecharacteristics of the reflecting information storage layer. Thus it isessential that each of the spectral components, comprising the compositelaser beam, be optically separated (i.e. isolated) from other spectralcomponents on the basis of both wavelength and polarization, andthereafter be channeled along a separate optical path for detection andoptical signal processing.

The function of light polarizing mechanism 9 is to impart a polarizationstate P_(j) to the incident laser beam according to alogical-to-physical address translation process carried out by addresstranslator 5, shown in FIG. 2. This system function can be carried outusing any one of a number of light polarizing mechanism well known inthe art.

The function of the laser beam collimating and shaping (i.e. optical)mechanism 10 is to collimate the spectral components produced by thelaser diodes and to then shape (i.e. circularize) the cross-sectionaldiameter of the composite laser beam for information reading operations.This system function can be carried out using one of a number of opticaldevices well known in the art, including collimating optics and aperturestops commonly used in laser beam profiling.

The collective function of the beam focusing mechanism 15, depthfocusing mechanism 12 and the auto-focusing/tracking mechanism 13 is tofocus the collimated laser beam to a depth within the multi-layerinformation storage device specified by depth information produced bythe address translator 5 and provided to the associated mechanisms bythe media access controller 6. Notably, the cross-sectional dimensionsof the focused laser beam must correspond with the physical spatialdimensions of each information storage cell realized in the opticalstorage device. Typically, these system functions are carried out usingan objective lens that focuses the laser beam to a small spot size onthe information storage media, while a mirror and lens subassembly aremounted in a moving coil actuation structure in order to providefocusing and tracking motion. Such mechanisms will be described ingreater detail hereinafter with reference to the particular embodimentshereof.

The function of the laser beam moving (i.e. steering) mechanism 11 is tomove the focused laser beam relative to the surface of the informationrecording device during information reading operations. This systemfunction can be carried out using electro-mechanical and/orelectro-optical mechanisms well known in the art for scanning focusedlaser beams along the coordinate directions of a prespecified coordinatesystem embedded within the optical storage device hereof.

The function of the storage media moving mechanism 17 is to move themulti-decked/multilayered information storage device 3 relative to thefocused laser beam during information reading operations. The manner inwhich this system function is carried out depends largely on the form inwhich the optical storage media is realized (e.g., disc, tape, card,etc.).

The function of the wavelength and polarization state detectionmechanism 19 is to optically isolate each spectral component in thereflected composite laser beam on the basis of both its wavelength andpolarization state. The function of the signal conversion mechanism 20then is to convert each of the individual spectral components that havebeen intensity-modulated (upon reflecting off the optical storagemedia), into a corresponding analog electrical signal for subsequentintensity-detection along a separate optical signal processing channelindexed by both spectral wavelength λ and polarization state P. Once theintensity modulation has been detected, it can be converted in a digitalvalue corresponding the information bit(s) recorded in the readinformation storage cell.

As will be shown in the various illustrative embodiments describedhereinbelow, the reflected laser beam is focused onto a small-areadetector, having a low capacitance and thus a high detection speed.Optically, the incident beam spot size at the information storage layerof the optical media is on order of the wavelength of the spectralcomponents of the laser beam. Typically, the beam spot isdiffraction-limited in order to achieve the highest areal densitypossible in each particular application. The laser beam reflected fromthe optical storage medium (i.e. by reflective diffraction due to thereflecting area being comparable to the wavelength of the incident laserbeam) is re-collimated by the same objective lens used for focusing, andsubsequently imaged by the focusing lens in front of the photodetector.Generally, the imaged reflection is magnified, as the size of thephotodetector typically much larger than the size of the focused beam atthe information storage medium.

To properly read information from a composite information storage deviceof the present invention, it is essential that particular items ofinformation be recorded on a set of prespecified tracks (e.g. called“Table of Contents”). The location of these information tracks can beset by standard, for example, on the top (i.e. uppermost) or bottom(i.e. lowermost) information storage layer of the device. Suchinformation items will typically include: information (i.e., digitalcode) identifying the optical media; information specifying the physicalparameters of the optical media; information specifying a directory offiles recorded on the information storage disc; information specifyingsubdirectories of files recorded on the information storage disc; thenumber of information storage decks in the device; the number ofinformation storage layers in each of these information storage decks;information regarding the physical arrangement with respect to eachother; and information regarding the characteristic wavelength andpolarization state of each specified information storage layer in thedevice.

During an information reading operation, the host computer typicallyrequests that a particular information file (e.g. document) be read fromthe information storage device hereof, and transmitted to the hostcomputer system by way of system bus 23. In order to carry out thissystem function, the address translation processor 5 illustrated in FIG.2 maintains two look-up tables. The data structure organization on theinformation storage disc is conventional: bits are grouped into bytes;bytes are grouped into sectors; and sectors are grouped into clusters.The first look-up table relates physical address (i.e. track) locationof each information storage “cluster” in the device, specified by thevector (D_(m), L_(n), T_(r), S_(q)), to the set of access parametersλ_(i), P_(j), x, y, z_(m,n)) used by the storage system controller 4 toread the corresponding information storage cluster location. The secondlook-up table relates the set of access parameters (λ_(i), P_(j), x, y,z_(m,n)) used by the storage system controller to read each informationstorage cluster location in the device, to the physical address locationof each information storage cluster therein, specified by the vector(D_(m), L_(n), T_(r), S_(q)). In the illustrative embodiment, theinformation recorded in a composite information storage device hereofcan be addressed by the information storage deck number D_(m),information storage layer number L_(n) in the deck, track number T_(r)on the information storage layer, and sector number S_(q) on the track.Such information items can be stored in the Table of Contents of thedevice using, for example, a 24-bit long information word. In such anillustrative embodiment, the deck index D_(m) can be represented by 3bits, corresponding to one of 2³ or 8 decks; the layer index L_(n) canbe represented by 5 bits, corresponding to one of 2⁵ or 32 layers ineach deck; the track index T_(r) can be represented by 8 bits,corresponding to one of 2⁸ or 128 tracks on each sector; and the sectorindex S_(q) can be represented by 8 bits, corresponding to one of 2⁸ or128 sectors on each layer. The data address (D_(m), L_(n), T_(r), S_(q))provides the physical address which is indicated by the laser wavelengthλ_(i) and polarization P_(j) to retrieve the data on sector S_(q) alongthe track T_(r) on the layer L_(n) in the deck D_(m).

The address transistor 5 may be realized as a separate hardware device(e.g. using ROM, PROM or like technology), it can be realized asfirmware within the storage system controller 4, or it can be realizedas software in the program storage system as in the case of a devicedriver. The data address (D_(m), L_(n), T_(r), S_(q)) produced by theaddress translator 5 is used by the storage controller to perform anumber of control functions. The first control function is to select aset of laser diodes having the set of characteristic wavelengthspecified by {λ_(i)}. The second function is to specify the set ofcorresponding characteristic polarization-states specified by {P_(j)}which must be imparted to the produced spectral components by way ofpolarization mechanism 9. The third function is to provide sufficientcontrol signals to laser moving mechanism 16 and storage media movingmechanism 17 in order that these functional subcomponents of the systemcan properly control steer the location of the incident composite laserbeam to information storage cells specified by either (x, y, z_(m,n)) or(r, θ, z_(m,n)), depending on the reference system used. The fourthfunction is to control depth focusing mechanism 12, servo-controllingmechanism 14 and auto-focusing/tracking mechanism 13 so that thespectral component of the composite laser beam having characteristicwavelength λ_(i) and polarization-state P_(j) is focused to theinformation storage cell located on the (D_(m), L_(n)) layer. As will beexplained in greater detail with reference to FIG. 10, this capacity tosimultaneously produce a laser beam with multiple spectral componentshaving different characteristic wavelengths and polarization-states,enables the system hereof to simultaneously read prerecorded informationin storage cells located in different information storage layers havingcorresponding characteristic wavelengths and polarization states.

As shown in FIGS. 3A and 3B, the information storage and accessingsystem of the present invention can be readily adapted to readinformation storage devices manufactured in the form of optical discs.The rotation of an optical disc of the present invention will depend onits type (i.e. how it was designed to be rotated and read duringinformation reading operations). Unlike conventional CD ROMs, in whichinformation tracks are formed along a continuous spiral and the opticaldisc must be rotated at Constant Linear Velocity (CLV), optical storagediscs hereof can be rotated at either a Constant Angular Velocity (CAV)or a Constant Linear Velocity. Due to the nature of the spiral trackalong which information bits are recorded in CLV type optical storagediscs hereof, the rotational speed of the disc, ω, depends on position(r,θ) of the information bit being read, as shown in FIG. 3C. For CLVtype discs, the servo-control information required to vary the angularspeed of the rotating disc as a function of information bit position (r,θ) is encoded within the information bits recorded on the optical discitself. In contrast, for CAV-type optical storage discs of the presentinvention, the angular velocity of the optical disc ω is maintainedconstant by a servo-mechanism, independent of the position (r, θ) of theinformation bits being read. In order to read both CLV and CAV typeoptical discs hereof, the information storage and accessing system ofFIGS. 3A and 3B includes an autodiscrimination mechanism forautomatically determining the type of optical storage disc being read,and suitable mechanism for accommodating the rotational requirementsthereof. When an optical storage disc is identified upon insertion intothe disc drive mechanism of the system of FIGS. 3A and 3B, prerecordedinformation about the disc type (e.g. stored on track 1) isautomatically read, decoded, and stored with the system controller, in amanner which will be described in greater detail with reference to thedisc process diagram of FIG. 23. While servo-control for disc rotationis a complex process, it nevertheless is well known in the art. Ingeneral, the CLC and chiral thin-film optical storage discs of thepresent invention can be realized as either CLV or ZCAV type discs. Whenrealized as CLV-type optical storage discs, the servo-control mechanismused for the conventional CD-ROM devices can be used to controlling theangular velocity of CLV-type optical storage discs. When realized aseither ZCAV or CAV type optical discs, the disc velocity control issimpler, as the angular velocity need only be maintained constant at asingle speed during information reading operations. The servo-controlmechanism for such types of optical storage discs can be constructed byembedding within suitable phase-locked loop (PLL) circuitry, either anoptical or magnetic encoder used to encode the rotational position (andthus velocity) of spindle of the motor used to rotate the opticalstorage disc.

For ZCAV type optical storage discs, the access time is inherently fast.The reason for such performance is due to the fact that the disk rotatesat a constant angular velocity and the access time is limited solely bythe movement of optical pickup. The ZCAV information storage formatexploits the increased information storage capacity of tracks radiallyoutward from the center of disc rotation. This is achieved by dividing(i.e. partitioning) the concentric tracks at the outer radii into moreinformation storage sectors, thereby increasing the information storagecapacity of the optical storage disc, while retaining the fast accesstime of the conventional CAV technique. Because the storage media isrotated at a constant angular velocity, the ZCAV format facilitatesmultiple optical pickups for data storage and retrieval, therebyincreasing the information transfer throughput of the system. While theZCAV format is preferred for information storage and accessing systemshaving high data transfer throughput, the CAV format may nevertheless beused with the optical storage media of the present invention and obtainexcellent performance results. Unlike CLV-type discs, the ZCAV formatallows for the use of multiple optical read pickups.

As shown in FIG. 3D, multiple optical pick-ups are assembled about aZCAV-type optical storage disc of the present invention supported upon amotor driven spindle operated under servo-control. Each optical readpick-up in this pick-up assembly is provided with its own independentservo-controls for tracking and focusing. Using a parallel array of koptical read pick-ups, as shown in FIG. 3D, the information transferrate can be increased k-fold, where k is the number of the pickups. In apreferred embodiment, the spectral range {λ₁, . . . , λ_(N)} is dividedin to k subranges and each optical pick-up is designed for particularspectral subrange. In the illustrative embodiments, the movement of eachoptical information pickup is constrained to follow an arc-like pathduring information reading operations. Alternatively, however, theoptical pickups can be designated to move along a radial directionduring information reading operations.

As shown in FIGS. 4A, 4B, 4C, 5A and 5B, the information storage andaccessing system of the present invention can be readily adapted to readinformation storage devices manufactured in either the form of opticalcards or optical tape. When manufactured in accordance with the presentinvention, these optical cards and tapes will comprise multipleinformation storage decks and layers, as generally described above. Inthe case of optical tape, it is understood that the optical storagemedia will be flexible to allow it to be transported using conventionaltape-transport mechanisms. The major modifications required in practicethese alternative embodiments of the present invention include providingsuitable means for transporting either composite laser beam relative tothe optical storage media or the optical storage media relative to thecomposite laser beam. In some embodiment, it may be desired to transportboth relative to each other. Such technical details are well within thescope of knowledge of those with ordinary skill in the art and thusshall not require further elaboration.

As illustrated in FIG. 6, the optical storage media of the presentinvention may generally comprise M information storage decks. In turn,each information storage deck may generally comprise N pairs ofinformation storage layers. Thus, the generalized embodiment of theoptical storage media of the present invention comprises 2MN informationstorage layers. Each such information storage pair is realized as aninformation storage structure having light reflective properties whichare dependent on both the wavelength and polarization state of theincident light beam employed during information reading operations. Ingeneral, the information storage decks are separated by free space, orby some optically isotropic material presenting no space interveningtherebetween.

As will be described in greater detail hereinafter, each informationstorage cell in which a prespecified information value (e.g. a logical“1”) has been recorded, has a characteristic wavelength λ_(i) andpolarization state P_(j) which reflects the spectral component of anincident laser beam having (i) its spectral band centered at awavelength approximately (i.e. substantially) equal to thecharacteristic wavelength λ_(i) of the recorded information storagecell, and (ii) its polarization state the same as the polarization stateP_(j) of the recorded information storage cell. These particularproperties of the optical storage media of the present invention enablethe recording and storage of discrete information states in the opticalstorage media. Similarly, each information storage cell in which adifferent prespecified discrete information storage value (e.g. logical“0”) has been recorded, is designed (i.e. constructed) to lack suchselective light reflective properties and thus be optically transparentto all other spectral components. Using this approach to informationstate recording/storage, information state detection mechanisms can beconstructed, as will be described in great detail hereinafter.

As indicated in FIG. 6, each information storage layer is labeled by(λ_(i), P_(j), D_(m), L_(n)), corresponding to the information storagelayer located within Deck m, and realized in information storage Layern. This specified information storage layer has a characteristicreflection band for the polarization state P_(j) centered approximatelyat wavelength λ_(i). Each n-th paired information storage structure hasa characteristic wavelength, while its pair of constituent layers(λ_(i), P₁, D_(m), L_(n)) and (λ_(i), P₂, D_(m), L_(n)) havepolarization states P₁ and P₂ that are orthogonal (i.e. perpendicular).As each n-th paired information storage structure comprises twowavelength and polarization-state selective layers as identified above,each m-th information storage deck may comprise up to 2N informationstorage layers.

According to the general principles of the present invention, digitalinformation recorded in the information storage cells of eachinformation storage layer (λ_(i), P_(j), D_(m), L_(n)) can be read usinga laser beam having a spectral component with a characteristicwavelength λ_(i) and polarization-state P_(j) that is identical to thecharacteristic wavelength λ_(i) and polarization-state P_(j) of theinformation storage layer (λ_(i), P_(j), D_(m), L_(n)). Thus, the 2MNinformation storage layers of the generalized embodiment requires only Nspectral laser lines in order to read out the stored information fromthe information storage cells in the various information storage layers.

Notably, the above specification of the information storage layers inthe optical storage media is provided solely for the pedagogicalpurposes, as the partial physical address (D_(m), L_(n)) is sufficientto specify the characteristic wavelength and polarization state (λ_(i),P_(j)) required by the address translator for information retrievaloperations. Also, it should be noted that the paired information storagelayers (λ_(i), P₁, D_(m), L_(n)) and (λ_(i), P₂, D_(m), L_(n)) need notnecessarily be in the same physical order, or in physical proximity witheach other, as shown. Furthermore, it is understood that while there maybe practical reasons for doing so in particular applications, eachpaired information storage structure need not be physically organizedaccording to increasing or decreasing characteristic wavelengths, asshown in FIG. 6.

Having described the overall structure and function of the informationstorage and retrieval system of the present invention, it is appropriateat this juncture to describe in greater detail various illustrativeembodiments of the novel information storage structured of the presentinvention.

In FIG. 7A, the first illustrative embodiment of the pairedwavelength-selective and polarization-selective information storagestructure of the present invention is shown. This structure can be usedto construct any one of many embodiments of the optical storage mediahereof. As shown, each paired information storage structure comprises: afirst planar information storage layer (λ_(i), P₁, D_(m), L_(n)) whichreflects incident light having polarization state P₁ and wavelengthλ_(i) wherever a binary bit ‘1’ is written (i.e. a first informationvalue storage location) and transmits incident light wherever a binarybit ‘0’ is written (i.e. a second information value storage location)regardless of the wavelength and polarization state of the incidentlight beam; a second planar information storage layer (λ_(i), P₂, D_(m),L_(n)) which reflects light having polarization state P₂ and wavelengthλ_(i) wherever a binary bit ‘1’ written and transmits incident lightwherever a binary bit ‘0’ is written regardless of the wavelength andpolarization state of the incident light beam; and a superstrate, asubstrate, and a plurality of interlayers between which the first andsecond planar information storage layers. In general, polarizationstates P₁ and P₂ of the incident laser beam are two mutually orthogonalto each other.

In general, Fresnel reflections should be eliminated in order to reducebackground noise during laser beam reception. The reflections whichwould otherwise occur at the air-superstrate of air-substrate interfacesof the optical storage device can be effectively minimized by coatingsuch surfaces with an anti-reflection (AR) coating. Optionally, thesubstrate-air interface can also be painted optically black (i.e. coatedwith a light-absorbing thin-film material). The reflections which occurat the interfaces of the plurality of interlayers and the informationstorage layers can be effectively minimized by selecting the refractiveindex of the plurality of interlayers to closely match the refractive ofthe information storage layers. By doing so, it is reasonable to neglectthe effects of Fresnel reflections which occur at interfaces of theinformation storage layers as the incident laser beam propagates towardsits addressed information storage layer during information retrievaloperations. Alternatively, in some embodiments and coatings may beavoided by carefully designing the optical system used to pick-upspectral components reflected from the optical storage media duringinformation reading operations.

Referring to FIG. 7A, on the top information storage layer, the shadedregions or areas (i.e. where a logical ‘1’ is stored) reflectsP₁-polarized light centered at a characteristic wavelength λ_(i)(P₁)with a bandwith of Δλ(λ_(i), P₁, D_(m), L_(n)), and transmits freelyP₁-polarized light having wavelengths outside this narrow reflectiveband and P₂-polarized light regardless of its wavelength. Thisstructure, for convenience, is called the (λ_(i), P₁, D_(m),L_(n))-layer. Similarly, on the bottom information storage layer, calledthe (λ_(i), P₂, D_(m), L_(n+1))-layer, the shaded areas where ‘1’ isstored reflects P₂-polarized light centered at a characteristicwavelength λ_(i)(P₂) with a bandwidth of Δλ(λ_(i), P₂, D_(m), L_(n+1))and transmits freely P₂-polarized light outside this reflective band andP₁-polarized light regardless of its wavelength. The clear areas on bothstorage layers transmits light freely, regardless the wavelength andpolarization of the incident light. As indicated earlier, P₁ and P₂ aremutually orthogonal polarizations, either circular or linear. Tooptically read out the stored data, the wavelength of laser beam λ_(i)is within the reflective band of both layers, as illustrated in FIGS. 7Band 7C. The two storage layers may have different characteristicwavelength and bandwidth. Ideally, the reflective bandwidth is matchedto the data readout optics which usually uses a focusing objective lenswith a high numerical aperture, among other considerations, such astemperature effects. Typical spectral widths of semiconductor laserdiodes used to read out the stored data are much narrower than thereflective bandwidth. To improve manufacturing, the characteristicwavelength and bandwidth of the paired structures can be slightlydifferent, that is λ_(i)≅λ_(i)(P₁)≅λ_(i)(P₂) and Δλ(λ_(i),P₁, D_(m),L_(n))≅Δλ(λ_(i), P₂, D_(m), L_(n+1)).

The light used to read stored information is assumed to be linearlypolarized or unpolarized. For linear polarization selective storagelayers, the linearly polarization of the light is further assumed to beat 45° with respect to the P₁ or P₂ direction. The incident light beamthen has spectral components P_(1(x)) and P_(2(y)) of equal intensity.For circular polarization-selective storage layers, the orientation ofthe linearly polarized incident light is not important, as it comprisesboth P₁ (i.e. right-handed circularly polarized, RHCP) and P₂ (i.e.left-handed circularly polarized, LHCP) spectral components of equalintensity. For unpolarized light, the incident laser beam has P₁ and P₂spectral components which are either linearly or circularly polarizedand having equal intensity.

As indicated in the table of FIG. 7D, different events occur when alaser beam is incident upon different regions of the paired informationstorage structure during information reading operations. For example,when the laser beam is incident on the paired information storagestructure at location “a”, where a ‘1’ information bit is recorded inboth the information storage layers (λ_(i), P₁, D_(m), L_(n)) and(λ_(i), P₂, D_(m), L_(n+1)), then the laser beam reflected from thepaired information storage structure has both P₁ and P₂ spectralcomponents. When the laser beam is incident on the paired informationstorage structure at location “b”, where a ‘0’ information bit isrecorded in both the information storage layers, then there is no laserbeam reflected from the paired information storage structure. When thelaser beam is incident on the paired information storage structure atlocation “c”, where a ‘1’ information bit is recorded in the (λ_(i), P₁,D_(m), L_(n)) layer and a ‘0’ information bit is recorded in the (λ_(i),P₂, D_(m), L_(n+1)) layer, then the reflected laser beam only has the P₁spectral component. When the laser beam is incident on the pairedinformation storage structure at location “d”, where a ‘0’ informationbit is recorded in the (λ_(i), P₁, D_(m), L_(n)) layer and a ‘1’information bit is recorded in the (λ_(i), P₂, D_(m), L_(n+1)) layer,then the reflected laser beam only has the P₂ spectral component.Notably, the information bit patterns shown in FIG. 7D are simplyillustrative; any type of discrete information pattern may be recorded,stored and retrieved using the principles of invention herein disclosed.Also, it is understood that in linear polarization-selective media, theincident polarization state of the incident need not be disposed at 45°with respect to the P₁ direction of the optical media. The incidentlight beam can be polarized in a direction that P₁ and P₂ componentshave a sufficiently intensity.

In typical embodiments, the cross-sectional dimension of the focusedlaser beam will be less than the physical dimensions of the lightreflecting and transparent regions of the information storage layers.Typically, these dimensions will be made microscopically small in orderto increase the information storage capacity of the optical storagemedia.

In FIG. 8A, a second illustrative embodiment of the paired informationstorage structure is shown. This wavelength and polarization selectivestructure can be used to construct any one of many embodiments of theoptical storage media hereof. This information storage structure issimilar to the structure shown in FIG. 7A, except that thenon-reflective material used to form the 0′ state information storagecells in FIG. 7A is replaced with an optically transparent materialsimilar to that used to fabricate the substrate or superstrate of thestructure of FIG. 7A. As shown in FIGS. 8B and 8C, the reflectancespectra of each of the planar information storage layers is centered(i.e. defined) about characteristic wavelength λ_(i). The Table of FIG.8D sets forth the polarization-state of the detected laser beam whilereading each of the illustrative information states at cell locations{a,b,c,d} recorded in the paired information storage structure shown inFIG. 8A.

In FIG. 9A, the third illustrative embodiment of the paired informationstorage structure of the present invention is shown. This pairedwavelength and polarization selective structure can be used to constructany one of many embodiments of the optical storage media hereof, e.g.,disc, tape or card storage format. Unlike the other illustrativeembodiments of the present invention, this paired wavelength-selectiveand polarization-selective information storage structure comprises: afirst undulated (i.e. stepped) information storage layer (Δλ_(i), P₁,D_(m), L_(n)) which reflects incident light (i.e. spectral components)having a wavelength within the characteristic wavelength band Δλ_(i) anda polarization state equal to the P₁-polarization state wherever abinary bit ‘1’ or a binary bit ‘0’ is written therein, and transmitsincident light (i.e. spectral components) having a wavelength fallingoutside of the characteristic wavelength band Δλ_(i) or a polarizationstate equal to the P₂-polarization state, which is orthogonal to theP₁-polarization state; a second undulated information storage layer(λ_(i), P₂, D_(m), L_(n)) which reflects incident light (i.e. spectralcomponents) having a wavelength within the characteristic wavelengthband Δλ_(i) and a polarization state equal to the P₂-polarization statewherever a binary bit ‘1’ or a binary bit ‘0’ is written therein, andtransmits incident light having a wavelength falling outside of thecharacteristic wavelength band Δλ_(i) or a polarization state equal tothe P₁-polarization state; and a plurality of interlayers disposedbetween the first and second undulated information storage layers, forsupport without light scattering. In general, polarization states P₁ andP₂ are two mutually orthogonal polarization states of spectralcomponents comprising the incident laser beam used to read informationpatterns recorded in the undulated surface of each information storagestructure. In this illustrative embodiment, both binary “1” and “0”states, recorded as high and low surface undulations (i.e. steps),reflect light having spectral components with a wavelength andpolarization state matched to the characteristic wavelength band andpolarization state of the information storage layer. Thus, a binary “1”recorded on a first undulated (i.e. stepped) information storage layer(Δλ_(i), P₁, D_(m), L_(n)) will produce a reflected signal that willhave a slightly higher light intensity when detected by thephotodetector of the optical pick-up used in the information retrievalsystem, whereas a binary “o” recorded on the same undulated (i.e.stepped) information storage layer (Δλ_(i), P₁, D_(m), L_(n)) willproduce a reflected signal that will have a slightly lower lightintensity when detected by the photodetector of the optical pick-up usedin the information retrieval system. On the basis of the difference indetected light intensity at the optical pick-up, the system is capableof determining whether a binary “1” or binary “0” has been recorded in aparticular storage cell location on such an information storage device.This technique in intensity level discrimination is used in conventionalCD-ROM reading units. As shown in FIGS. 9B and 9C, the reflectancespectra of each of the planar information storage layers is anarrow-band Δλ_(i) centered substantially about characteristicwavelength λ_(i). The Table of FIG. 9D sets forth the polarization-stateof the detected laser beam while reading each of the illustrativeinformation states at cell locations {a,b,c,d} recorded in the pairedinformation storage structure shown in FIG. 9A.

In FIG. 9E, a fourth illustrative embodiment of the paired informationstorage structure is shown. This wavelength and polarization selectivestructure can be used to construct any one of many embodiments of theoptical storage media hereof. This information storage structure issimilar to the structure shown in FIG. 9A, except that the reflectivematerial used to form the “1” state information storage cells in FIG. 9Ais thin, artificial chiral film. Methods for fabricating thisinformation storage structure are illustrated in FIGS. 33A through 33G.As shown in FIGS. 9F and 9G, the reflectance spectra of each of theplanar information storage layers is centered (i.e. defined) aboutcharacteristic wavelength λ_(i). The Table of FIG. 9H sets forth thepolarization-state of the detected laser beam while reading each of theillustrative information states at cell locations {a,b,c,d} recorded inthe paired information storage structure shown in FIG. 9E.

In FIG. 13A, the fifth illustrative embodiment of the paired opticalstorage structure of the present invention is shown. This particularembodiment is based on the light reflectance properties of CLC materialwhich are dependent on both the wavelength and circularpolarization-state of the incident light. This paired informationstorage structure can be used to construct the numerous embodiments ofthe optical storage media hereof. As shown, the paired informationstorage structure comprises: a first planar information storage layer(i.e. RH-layer) which reflects incident light having a right-handedcircularly polarized (RHCP) state and wavelength λ_(i)wherever a binarybit ‘1’ is written and transmits incident light wherever a binary bit‘0’ is written regardless of polarization and wavelength; a secondplanar information storage layer (i.e. LH-layer) which reflects incidentlight having a left-hand circularly polarized (LHCP) state andwavelength λ_(i) wherever a binary bit ‘1’ is written and transmitsincident light wherever a binary bit ‘0’ is written regardless ofpolarization and wavelengths; a superstrate, a substrate and a pluralityof interlayers disposed between the first and second planar informationstorage layers.

CLC material reflecting RHCP light is called a dextrorotary orright-handed (RH) CLC and, similarly, the CLC reflecting LHCP light iscalled a levorotray or left-handed (RH) CLC. Reflectance nearly 100% canbe obtained with a layer of CLC material only a few microns thick. Forinformation storage applications of the present invention, such highreflectance is not necessary. In fact, for decked information storagemedia of the present invention, a lower reflectance is prescribed. Thereflectance of CLC has a spectral bandwidth given by Δλ≈λ₀Δn/n_(av),where Δn is the birefringence, and n_(av) is the average refractiveindex and is centered at the characteristic wavelength λ₀. The lightreflectance R of polymer CLC material is determined by the CLC layerthickness L and is governed by the equation R=tanh²[(Δn/n_(av))(L/P)],where the ratio L/P is an integer corresponding to the number of helicalpitches in the film. Details concerning the phenomena are described atgreater length in the publication entitled ‘Design and construction of1064-nm liquid-crystal laser cavity end mirrors, by J.C. Lee and S.D.Jacobs, published in Journal of Applied Physics, Vol. 68, No. 274(1990).

The characteristic wavelength λ₀ of CLC material is given by n_(av)Pwhere P is the helical pitch associated with the particular CLC. Ingeneral there are two known ways of tuning the characteristic wavelengthof CLC material. In particular, the characteristic wavelength of CLCpolymer can be tuned by controlling the concentration of chiraladditive, as described in the following publications: U.S. Pat. No.5,353,247 to Faris; ‘Cholesteric Structure I: Optical Publications’ inLiquid Crystals, by J.L. Fergason, published at page 89, with G.H.Brown, G.J. Dienes and M.M. Labes eds., (Gordon & Breach, New York,1967); and ‘Liquid crystal laser optics: design, fabrication andperformance,’ by S.D. Jacobs et al, in Journal of the Optical Society ofAmerica, Vol. B5, No. 91962 (1988). The characteristic wavelength ofpolymer CLC material can also be tuned by controlling the curingtemperature, as described in the publication: ‘Liquid crystal side chainpolymers: properties and aspects for applications, by H. Finkelmann andH.J. Kock, published in Display Technology, Vol. 1, No. 81 (1985).

In general, there are four different states of molecular ordering forCLCs, namely: planar state; focal-conic state; homeotropic state; andisotropic state. While these states are described in the book “LiquidCrystals” cited above, it will be helpful to briefly review each suchstate of CLC molecular ordering below and indicate how the same can beused to realize the information storage values in the optical media ofthe present invention.

The planar state of molecular ordering its where CLC molecules areordered in a helix, having an optical axis pointing in the samedirection, typically normal to the face. Because of such molecularordering, CLC material in this state exhibits wavelength andcircular-polarization selective reflectance. In the present invention,the planar state of CLC material can be used to realize a particularlogical state value (e.g. “1”) to be recorded in the optical storagemedia hereof, as it will produce a signal of higher value.

The focal-conic state of molecular ordering is where CLC molecules areordered in a helical structure, but the helixes are randomly oriented.While each individual helix has wavelength and polarization selectivereflectance properties, light scatters in all directions, thus reducingthe reflected light in any particular direction. Therefore, in thepresent invention, the focal-conic state of CLC material can be used torealize a particular logical state value (e.g. “0”) to be recorded inthe optical storage media hereof, as it will produce a signal of lowervalue.

The homeotropic state of molecular ordering is where CLC molecules arenot ordered in a helix. The CLC molecules are rod-shaped, and have aoptical axis pointing in one direction which is typically normal to theface of the thin-film. Because of such molecular ordering, CLC materialin this state does not exhibit wavelength and circular-polarizationselective reflectance. Therefore, in the present invention, thehomeotropic state of CLC material can be used to realize a particularlogical state value (e.g. “0”) to be recorded in the optical storagemedia hereof, as it will produce a signal of lower value.

The isotropic state of molecular ordering is where the individual CLCmolecules have a random orientation. Actually, this state is not aliquid crystalline state because there is no molecular ordering. Thus,CLC matter is said to be an isotropic state if its temperature is raisedabove a characteristic (i.e. clearing) temperature. Apart from Fresnelreflection, CLC material in the isotropic state does not exhibitwavelength and circular-polarization selective reflectance. Therefore,in the present invention, the isotropic state of CLC material can beused to realize a particular logical state value (e.g. “0”) to berecorded in the optical storage media hereof, as it will produce asignal of lower value.

Notably, the above-described states of polymer CLCs provide severalimportant advantages when fabricating the optical storage media of thepresent invention. In particular, the first information value storagecell locations (e.g. logical “1s”) can be recorded first in aninformation storage structure hereof by inducing the CLC material intoits planar state (e.g. using shearing or field alignment techniques).Thereafter, the second information storage value cell locations (e.g.logical “0”) can be recorded by (i) raising the temperature ofparticular portions of the CLC material to its “clear temperature” inorder to cause it become isotropic, or (ii) subjecting particularportions thereof to a strong electric or magnetic field in order tocause it become focal-conic or homeotropic. Thereafter, the processedCLC material can be polymerized using conventional techniques and thenallowed to cool so that the ordering of the CLC molecules are fixed orfrozen. An alternative technique for realizing the logical “0” state isby destroying the CLC molecules.

According to the technical teachings on the present invention set forthabove, polarization states P₁ and P₂ in the CLC-based informationstorage structure are RHCP and LHCP, respectively. Without loss ofgenerality, each first information value (e.g. logical ‘1’ value)recorded in the top CLC-based information storage layer reflectsincident light having (i) a spectral bandwidth of Δλ(λ_(i), RH, D_(m),L_(n)) centered at a characteristic wavelength λ_(i)(RH) and (ii) a RHCPstate. For convenience, this information storage layer is denoted as the(λ_(i), RH, D_(m), L_(n))-layer. In contrast each first informationvalue (e.g. logical ‘1’ value) recorded in the bottom CLC-basedinformation storage layer reflects incident light having (i) a spectralbandwidth of Δλ(λ_(i), LH, D_(m), L_(n)) centered at a characteristicwavelength λ_(i)(LH) and (ii) a LHCP state. This information storagelayer is called the (λ_(i), LH, D_(m), L_(n+1))-layer. Together, thepaired CLC layers are fabricated such that characteristic wavelengthλ_(i)(RH)≈λ_(i)(LH) and the spectral bandwidth Δλ(λ_(i), RH, D_(m),L_(n))≈Δλ(λ_(i), LH, D_(m), L_(n+1)). Importantly, the characteristicwavelength of the incident laser beam component, λ_(i), is selected sothat it lies within the reflection band of the corresponding pairedinformation storage structure, i.e., λ_(i)≈λ_(i)(RH)≈λ_(i)(LH). As shownin FIGS. 13B and 13C, the reflectance spectra of each of the planarinformation storage layers is centered about characteristic wavelengthλ_(i). The Table of FIG. 13D sets forth the polarization-state of thedetected laser beam while reading each of the illustrative informationstates at cell locations {a,b,c,d} recorded in the paired informationstorage structure shown in FIG. 13A. In this illustrative embodiment ofthe optical storage media hereof, the information storage cellsspecifically designed to storing particular information values (e.g.logical “1” values) by selectively reflecting from the pairedinformation storage layers (λ_(i), RH, D_(m), L_(n)) and (λ_(i), LH,D_(m), L_(n)), only incident light having particular characteristics,can be fabricated from either different CLC materials, or from the samenematic liquid crystal material with chiral additives of oppositechirality.

Using the above-disclosed teachings, paired CLC-based informationstorage structures having (λ_(i), LH, D_(m), L_(n+1)) and (λ_(i), RH,D_(m), L_(n+1)) information storage layers can be made usingcommercially available materials. For example, LH-type CLC material witha prespecified characteristic wavelength λ_(i) can be made using LH-typeCLCs available from Wacker Chemie GmbH, of Munich, Germany, identifiedby Wacker Catalogue Numbers CC4039L and CC4070L, each having acharacteristic wavelength of 390 nm and 700 nm, respectively. By mixingthese cross-linkable CLCs together, a LH-type CLC material can beproduced having a characteristic wavelength adjustable between 390 nm to700 nm. The empircal formula for the production of such LH-type CLCmaterial is given by λ_(i)=1000/(1.49+1.15 c), where c denotes theconcentration of CC4039L in CC4070L. The characteristic wavelength λ_(i)can be finely tuned by adjusting the cross-linking temperature.Similarly, RH CLC material with a prespecified characteristic wavelengthλ_(i) can be made using cross-linkable CLCs identified by WackerCatalogue Numbers CC4039L and CC4039R each having characteristic awavelength of 390 nm, and Wacker Catalogue Number CC4070L having acharacteristic wavelength of 700 nm. By mixing together the RH-type andLH-type CLCs, either a RH-type or a LH-type CLC material can beobtained, depending on the relative concentrations of the twocomponents. For these cross-linkable CLCs, the characteristic wavelengththereof can be finely tuned by adjusting the cross-linking temperature.The above technique is well known and described in greater detail in“Liquid Crystals” (1992) by S. Chandrasekhar, published by CambridgeUniversity Press. Having produced LH and RH CLC materials as describedabove, CLC optical storage devices can then be made using any one of thestorage device manufacturing techniques described hereinafter. Forexample, according to one such manufacturing technique, the LH-type CLCmaterial can be used to fill in a first microscopic pit pattern formedin one side of optically transparent substrate, whereas the RH-type CLCmaterial can be used to fill in a second microscopic pit pattern formedin the other side of the optically transparent substate. Once these CLCmaterials have been applied to the substrate, electric fields, magneticfields, or mechanical shear forces can be used to align the CLC into aplanar (i.e. reflective) state. Thereafter, various types of curing andlaminating techniques can be practiced.

Following the teachings set forth in FIG. 6, the paired CLC informationstorage structure can be stacked in one or more information storagedecks in order to increase the information storage capacity of theresulting CLC storage media. Thus with a information storage mediahaving M decks fabricated from CLC material, it is possible to read 2MNinformation storage layers using only N laser lines (i.e. spectralcomponents). Consequently, this provides the generalized system of thepresent invention with a 2M-fold increase in information storagecapacity over prior art systems.

Referring to FIG. 14, there is shown a decked information storage andretrieval system based on the CLC-based storage media described above.In this illustrative embodiment of the present invention, only a singlespectral component is employed to construct each and every informationstorage deck in this multi-deck system (i.e. N=1) and therefore a singlelaser with the characteristic wavelength of the CLC material of theoptical storage media is required to read out information prerecorded inthe 2M information storage layers of the optical storage media of thesystem. When compared to the conventional technique of stackingsemi-reflective surfaces of conventional CD media, the paired CLCstorage media of FIG. 14 provides a doubling in storage capacity, otherthings being equal, namely: the number of stacked layers (i.e., M), thecharacteristic wavelength of the laser reading beam, and the media formfactor. This information storage capacity of the system of FIG. 14 canbe greatly increased by simply stacking N paired RH-LH informationstorage layers within each of the M information storage decks. In such amodified system, N laser lines (i.e., spectral components) will berequired to read the 2MN CLC information storage layers in the resultingCLC-based information storage media. Again, when comparing the somodified system of FIG. 14 to the prior art stacked CD storage media, an2N-fold increase in information capacity is realized.

The CLC storage media described above exploits the wavelength andcircular-polarization selective reflection properties of cholestericliquid crystals which naturally arise from the long-range molecularordering of organic molecules. However, the optical storage media of thepresent invention may also be realized using synthetic light reflectivecoatings (e.g., films) which exhibit a wavelength-dependent circulardichroism similar to CLC material. Such films, for example, can beobtained by vapor deposition on a rotating substrate at an obliqueangle, as described in the publication ‘Chiral thin solid films: methodof deposition and applications’, by R.M.A. Azzam, in Applied PhysicsLetters, Vol. 61, 3118(1992). Physically, each birefringent thin-filmlayer of a few angstroms thick is an analog to a single CLC molecularlayer which is also of a few angstroms thick. By rotating the substrate,the optic axis of the deposited birefringent film rotates continuouslyin the same rotation direction of the substrate. As the depositionaccumulated, the optical axis traces a helical pattern along the z-axis.The structural analogy to cholesteric liquid crystal becomes apparent,leading to the similar wavelength and polarization selectivereflectance. The chirality, i.e., RHCP and LHCP of the film, is set bythe direction of the substrate rotation, which determines the rotationsense of optical axis. The helical pitch P is controlled by the rate ofdeposition per rotation. The film birefringence Δn is controlled by theoblique angle of deposition and the material itself. Similar to theCLCs, the spectral reflectance can be controlled by the number ofpitches, where one complete rotation about the optic axis equals onepitch. The wavelength and circular polarization selective reflectivityof this artificial chiral film follows the same formulas for the CLCmaterials.

Referring to FIGS. 15A and 15D, a method will be described forfabricating a pre-recorded information storage layer usingvacuum-deposited chiral thin-film (e.g. vacuum-deposited inorganic oxidethin film) having wavelength and polarization selective reflectanceproperties that are applied in optical storage applications of thepresent invention. As indicated at FIG. 15A, the first step of themethod involves producing a stamped disc having a undulated (i.e.pitted) surface similar to conventional CD discs. Notably, the surfaceundulations encode digital bits of information in a conventional manner.As indicated in FIG. 15B, the next step of the method involvesdepositing an artificial chiral coating or film over the stamped disk.In accordance with the principles of the present invention, thedeposited chiral film reflects either RHCP light or LHCP light atcharacteristic wavelength λ_(i), depending on the requirements of theinformation storage layer within the resulting optical media.Thereafter, the layer is coated with an optically transparent materialin order to planarize the layer, as shown in FIG. 15C, and therebyproduce an information storage layer of the present invention. Thiscoating step can be carried out by applying a polymer to the chiralthin-film coated disc and then spin-coated and allowed to cure.Thereafter, information storage layers produced in the manner describedabove can be then stacked as shown in FIG. 15D. Following the technicalteachings set forth in FIG. 6, multi-deck storage media can befabricated using the above-described chiral thin-film coatings.

In accordance with the general principles of the present invention, theoptical storage media hereof can be constructed using materials exhibitsboth wavelength and linear-polarization dependent reflection properties.One such way to produce such information storage layers is byconstructing multilayer thin-film structures of quarter-wavelengththickness with alternating high and low refractive indices. Thisembodiment of the present invention will be described in greater detailbelow with reference to FIG. 16.

In FIG. 16, the fifth illustrative embodiment of the information storagestructure hereof is shown. This embodiment is based on stackedmultilayer dielectric thin film material, which can be used to constructthe optical storage media hereof. Notably, this stacked birefringentmultilayer film material (i.e. having a different index of refractionalong its x and y axes) exhibits wavelength and linear-polarizationselective reflectance properties as hereinbefore described. This isunlike conventional multilayer dielectric mirror coatings which do notexhibit polarization selectivity, due to the fact that each dielectriclayer therein is isotropic. As shown in FIG. 16, the multilayerinformation storage structure comprises: a substrate having a refractiveindex n_(s), for mechanical support; first flat information storagelayer (λ_(i), P₁, D_(m), L_(n)), realized as as first set of dielectricthin-film layers, which reflects incident light having a first linearlypolarized state P₁ and wavelength λ_(i) wherever a binary bit ‘1’ iswritten and transmits incident light wherever a binary bit ‘0’ iswritten regardless of polarization and wavelength; a second flatinformation storage layer (λ_(i), P₂, D_(n), L_(n)), realized as asecond layer of dielectric thin-film layers, which reflects incidentlight having a second linearly polarized state P₂ (orthogonal to P₁) andwavelength λ_(i) wherever a binary bit ‘1’ is written and transmitsincident light wherever a binary bit ‘0’ is written regardless ofpolarization and wavelength; and an optically transparent top layerhaving a refractive index n_(c), which functions as a protective dustcover.

In the preferred embodiment of the optical storage media of FIG. 16, themultilayer structure disposed between the substrate and the dust coverconsists of k alternating thin-film layers with refractive index n₁ andbirefringent layers with n_(2x)=n₁ and n_(2y) which is spatiallymodulated on a microscopic scale. Light polarized along x-axis transmitsfreely as the refractive indices are matched. Light polarized alongy-axis transmits freely where n_(2y)=n₁ (to represent bit ‘0’) andreflects where n_(2y)≢n₁ (to represent bit ‘1’). The maximum lightreflectance R is for the quarter-wavelength stack (n_(2y)t₂=n₁t₁=λ_(i)/4) which is given byR={[n_(c)/n_(s)−(n_(2y)/n₁)^(2k)]/[n_(c)/n_(s)+(n_(2y)/n₁)]^(2k)}².Birefringent films can be obtained by exposing dye-doped polymers withlinearly polarized light. For example, a large stable Δn≈0.2 forpolyester azo side groups has been reported in the publication ‘Largephotoinduced birefringence in an optically nonlinear polyester polymer,by Y. Shi et al. in Applied Physics Letters, Vol. 59, 2935 (1991).Designs not based on quarter-wave stacks are described in ‘Coatings andfilters, by J.A. Dobrowolski in Handbook of Optics, W.G. Driscoll and W.Vaughan eds., p8-65, McGraw Hill Book Co., New York, 1978. According tothe technical teachings of the present application, this is theP₂-storage layer. Using the information storage structure describedabove, it is possible to construct a multi-decked optical storage mediahaving both wavelength and linear polarization-state dependentreflection properties. While this optical media is preferred in theconstruction of card and type information storage devices, it can alsobe used to construct compact disc type devices as well.

Referring to FIG. 10A, there is shown an information storage andretrieval system which uses optical storage media that permitssimultaneous reading of information stored in different informationstorage layers. As shown, the optical storage media is constructed fromtwo paired information storage structures, having characteristicwavelengths λ₁ and λ₂ and polarization-states P₁ and P₂, respectively.FIGS. 10B, 10C, 10D, and 10E provide a schematic representation of thereflectance spectra of each of the information storage layers shown inFIG. 10A. Using a laser beam having spectral components withcharacteristic wavelengths λ₁ and λ₂ and polarization of P₁ and P₂, asindicated in FIGS. 10B, 10C, 10D and 10E, information stored in each ofthese paired information storage structures can be simultaneously readduring information reading operations.

Following the nomenclature of naming the two-layered P₁-P₂ structure asdiscussed above, the (λ_(i), P₁, D_(m), L_(n))-layer is a layer reflectsa P₁-polarized light of wavelength λ_(i) with a bandwidth of Δλ(λ_(i),P₁, D_(m), L_(n)) where bit ‘1’ is written. It is required thatΔλ(λ_(i), P₁, D_(m), L_(n))≈Δλ(λ_(i), P₂, D_(m), L_(n+1)) and the outputwavelength of readout laser λ_(i) be within the correspondingreflectance bands for i=1, 2, . . . , N. For minimum signal cross-talkbetween different information storage layers, the reflection bands arenot overlapped as illustrated in FIGS. 10B, 10C, 10D and 10E for thecase where N=2.

In FIG. 11B, schematic representation of the simplest form of a deckedinformation storage system (i.e. were M=2) is shown. As illustrated,each information storage deck consists of two paired P₁-P₂ informationstorage structures of the type shown in FIGS. 7A, 7B, 7C, 8A, 8B, 8C,9A, 9B and 9C. In such a system, prerecorded information in each pair ofinformation storage structures can be read using a single focused laserbeam having a characteristic wavelength λ_(i) and a polarization statewhich corresponds to the characteristic polarization state of theparticular layer being read. The advantage of such a system becomesobvious: only one optical pickup with one laser is required to readinformation from all four information storage layers, thereby increasingthe information storage capacity of such a system by a factor of four.The accessing, tracking and focusing of each deck is same as for asingle deck system. The focusing objective lens is moved up or down inorder to access the particular deck. As will be explained in greaterdetail, the disk-drive can recognize this type of multi-decked media byspecial coding on a particular track on the media.

As shown in FIGS. 11B and 11C, the spherical aberration compensator 24is an optically transmissive plate of variable thickness. It is placedbetween the focusing means 1101 and the information storage media. Thepurpose of the spherical aberration compensator 24 is to maintainconstant optical distance between the focusing means 15 and theinformation layer being readout from the optical storage media 3.Without this corrector or compensator, the focused laser beam spot willnot be diffraction-limited, due to spherical aberration. Notably, thethickness of the compensator can be adjusted continuously or in discretesteps, according to the location of the information layer. As shown ingreater detail in FIG. 1, the thickness control signal is provided bythe storage system controller 22. As shown in FIGS. 11B and 11C, thecompensator of the illustrative embodiment is realized as a cell 1102having windows surfaces 1103 and 1104, which are transmissive over thespectral range of the laser beam used during information readingoperations. The cell is filled with an optically clear fluid 1105 havingan index of refraction typically in the range of 1.4 to 1.7. Notably, itmay be advantageous to use a fluid 1105 with a higher index ofrefraction, as this will allow cell 1102 to be made thinner, as would bedesired in many applications. In response to thickness control signalsproduced by system controller 22, one or both of the window surfaces1103 and/or 1104 are designed to move up and down like a piston, inorder that the thickness of the liquid within in the cell is adjustedsuch that the optical path length between the focusing means 1100 andparticular information layer (i.e.,(nt)_(liquid)+(nt)_(windows)+Σ(nt)_(information layers) above theparticular layer) is kept nearly constant. In FIG. 11B, the compensatoris shown configured for reading out information recorded from the toplayer 1106 of a multi-layer optical storage device hereof, whereas inFIG. 11B2, the compensator is shown configured for reading outinformation recorded from lower layer 1107 thereof. By compensating forspherical aberration using this compensation device, the focused beamspot incident upon the information storage layer to be read isdiffraction-limited, a requirement for precise reading of high-densityinformation recorded within different information storage layers of theoptical storage media hereof.

In FIGS. 12A and 12B, a schematic representation of a more complex,multi-decked information storage system is shown, in which M indicatesthe maximum number of information storage decks and N indicates themaximum number of pairs of information storage layers in an informationstorage deck. In the system, prerecorded information in each L_(n)-thinformation storage structure within each D_(m)-th information storagedeck can be read using the following system components: (i) a singlefocused laser beam having a characteristic wavelength λ_(i)corresponding to the characteristic wavelength of the particularinformation storage layer being accessed; (ii) an adjustable mechanismfor moving the focused laser beam up and down along a particularinformation storage deck being read; and (iii) and adjustable mechanismfor fine-focusing the incident laser beam onto particular theinformation storage layer within a particular information storage deck,and thereafter tracking the same during the information readingoperation.

A preferred design for the multi-deck storage system of FIGS. 12A and12B requires that the reflectance R_(m) for deck D_(m) satisfy thefollowing expression: R₁≈(1−R₁)²R₂≈(1−R₁)²(1−R₂)²≈ . . . ≈(1−R_(M-1))²R_(M), for m=1, 2, . . . , M. In this illustrative embodiment,it is assumed that the transmission coefficient T_(m)=1−R_(m). For thoseskilled, this condition can be modified accordingly if there aretransmission losses. In such instances, it is clear that R₁<R₂< . . .R_(M-1)<R_(M)<1 and R₁<1/M. For purposes of illustration, consider thecase of a five deck system where M=5. According to the above-defineddesign criteria, R₁≈12% (first/top deck), R₂≈16% (second deck), R₃≈23%(third deck), R₄≈38% (fourth deck) and R₅ ≈100% (fifth/bottom deck).Following such design criteria, the intensity of the reflected laserbeam from each deck will be about 12% of the intensity of the incidentlaser beam. By following the above design criteria for layer-reflectancewill assure that the signals received by the photodetector from eachdeck in the system are approximately equal. Notably, the reflectancecoefficient (i.e. R_(m)) can also be chosen that the signal-to-noiseratio (SNR) for each deck is above the minimum level required to achievea given bit error rate. As the reflected signal strength decreasesrapidly with increasing values of M, limits are naturally set on thevalue of SNR that can be obtained for any particular system design. Whenusing signal detection techniques commonly used in conventional CD-ROMsreading units, systems having five decks (i.e. M=5) can be easilyachieved. With improved signal detection techniques, systems withgreater than five information storage decks should also be realizable.

When designing multi-deck information storage systems according to thepresent invention, a number of factors, such as cross-deck noise and theworking distance of the laser beam focusing optics, present restrictionson the number of information storage decks that can be practicallyrealized in any particular information storage and retrieval system.

During information reading operations, “cross-deck noise” is created bythe reflection of laser light from the layers within other informationstorage decks in the optical storage device. A number of techniques maybe employed to adequately reduce the effect of cross-deck noise. Onetechnique involves increasing the optical distance between informationstorage layers in next-neighbor decks, having the same characteristicwavelength λ_(i) and polarization-state P_(j). Another techniqueinvolves ensuring that the optical distance between the substrate,interlayers and superstate (i.e. d_(i)) is 100 times greater than thedepth of focus of the laser beam detection optics. For a moderate numberof information storage layers (i.e. N), the deck thickness can begreater than the depth of focus. Another technique for minimizingcross-deck noise involves selecting distance d_(i) to be a random value,not a multiple of λ_(i)/2.

The working distance of the focusing objective lens of the laserdetection (i.e. pick-up) optics imposes another restriction on thenumber of decks through which the laser beam can read. The workingdistance can be increased by using a larger lens aperture, i.e.,diameter, for a given Number Aperture (N.A.). However, practical limitswill be set by the physical limitations of available technology. Forexample, when using the actuators of conventional CD-ROM drives fortacking and auto-focusing, constraints will be naturally imposed on boththe physical size and mass of the focusing objective lens of the laserbeam focusing/detection (i.e. pick-up) optics. Consequently, the workingdistance of the laser beam focusing/detection optics will be constrainedby such factors.

Having described the wavelength and polarization-state dependent storagestructures of the present invention, it is appropriate at this junctureto now describe in greater detail the laser focusing/detection subsystem(i.e. pick-up optics) schematically represented in FIG. 1 and describedgenerally hereinabove.

In general, there are two preferred techniques for reading informationfrom the optical storage media hereof using precharacterized laserbeams. The first technique, illustrated in FIG. 17, controls thepolarization-state of the spectral components of the laser beam as it isfocused incident on the optical storage media, and shall be referred tohereinafter as a “type-1” optical pick-up. The second technique,illustrated in FIG. 20, controls the polarization-state of the spectralcomponents of the reflected laser beam received by the photodetector,and shall be referred to hereinafter as a “type-2” optical pick-up.Notably, each of these optical pick-up subsystems is based onnon-retroreflective design principles in order to prevent opticalfeedback to the laser diodes. One disadvantage of such an optical designis that the aperture of the focusing objective lens in the opticalpick-up is not fully utilized.

As shown in FIG. 17A, the type-1 optical pick-up subsystem comprises anassembly of components, namely: a laser beam producing unit 1701 forproducing a collimated and circularized laser beam having one or morespectral components (i.e. lines) with characteristic wavelengths andpolarization-states specified by the media access controller; a pair ofbeam-steering mirrors 1702 and 1705, for steering the produced laserbeam after it has been collimated and circularized; a light diffractivephase-grating 1703 for diffracting the incident laser beam into threelight beams (i.e. 0-th, +1st, and −1st order diffractive beams) for usein beam tracking, wherein the light diffractive efficiency of thegrating over the 2nd and higher orders is minimized to conserve laserpower; a focusing objective lens 1704 for focusing the collimated laserbeam as it is directed incident to the optical storage media hereof; afirst ultra-compact housing 1708 within which phase-grating 1703 andobjective lens 1704 are mounted (i.e. enclosed) and which is moveable upand down under servo-control for fine focusing of the incident laserbeam during information reading operations; a second ultra-compacthousing 1709 with which beam-steering mirror 1702 and ultra-compacthousing 1708 are mounted and which is radially translatable (i.e. moved)under servo-control for controlling the tracking of the incident laserbeam during information reading operations; a spherical aberrationcompensator (i.e. corrector) 1707, as disclosed in FIGS. 11B and 11C;and an array of photodetectors 1706 for detecting the different narrowbands of spectral components of the laser beam as they reflect offcorresponding information storage layers in the optical storage mediaduring information reading operations.

In the illustrative embodiment(s), each information storage andretrieval system has two independent servo-control mechanisms. Thefunction of the first servo-control system is to control the movement of(i) the phase grating 1703 and objective lens 1704, for the purpose ofauto-focusing the incident laser beam onto a specified informationstorage layer. The function of the second servo-control mechanism is tocontrol the movement of the (ii) the assembly of the phase grating 1703,objective lens 1704, beam steering mirror 1702, and spherical aberrationcompensator 1707 relative to the information storage media, for thepurpose of controlling the incident laser along a specified informationtrack. The first servo-mechanism can employ a voice coil in order toeffect micro-movements required to control the location of the incidentlaser beam along an information track during information readingoperations. The second servo-mechanism can use another voice-coil or alead screw in order to effect larger movements required for controllingthe movement of the incident laser beam from one information recordingtrack to another on the optical storage disc of the present invention,as shown in FIG. 17I. While voice coils are presently the most populartranslational actuator used in such servo-control applications, it isunderstood that other actuators, based on piezoelectric principles, canalso be used to practice the present invention.

In FIG. 17B, the subcomponents of the laser beam producing unit areshown in greater detail. As shown, the laser beam producing unit 1701comprises: an array of lasers, e.g. lasers 170101, 170111 and 170191,for producing spectral components with characteristic output wavelengthsλ₁, λ_(i), and λ_(N), respectively, and a predetermined polarizationstate (e.g. linear polarization state); an array of collimating lenses,170102, 170112, and 170192, for collimating the spectral output oflasers 170101, 170111 and 170191, respectively; an array of anamorphicprisms, 170103, 170113 and 170193, for circularizing the spectral outputfrom collimating lenses, 170102, 170112, and 170192, respectively; anarray of quarter-wave (λ/4) plates 170104, 170114 and 170194, forconverting the linear polarized spectral components into, for example,RHCP spectral components; and array of voltage-controlled half-wave(λ/2) retardation plates 170105, 170115 and 170195, for converting RHCPspectral components into LHCP spectral components; and an array ofdichroic mirrors 170101, 170116 and 17096, respectively, for combiningthe polarized spectral components into a single composite laser beam,which has been precharacterized for reading particular informationstorage layers having characteristic wavelengths and polarization-stateswhich correspond to the wavelengths and polarization-states of thespectral components of the incident composite laser beam.

Having described the type-1 optical pick-up of the illustrativeembodiment, a number of modifications readily come to mind. For example,the lenses and prisms of the collimating and circularizing optics in theabove-described optical pick-up can be replaced by a pair of cylindricallenses of different focal lengths for collimating and circularizingoutput spectral components produced from the array of laser diodes.Also, instead of using the combination of the λ/4-plate and thevoltage-controlled λ/2-plate, a voltage-controlled λ/4-3λ/4phase-retardation plate can also be used in order to change linearlypolarized spectral components into RHCP or LHCP spectral components. Inaddition, instead of using an array of mirrors, a diffraction grating ora prism may be used to combine the polarized spectral components into asingle composite laser beam.

Preferably, the λ/4-plates of the type-1 optical pick-up described aboveare made from of birefringent crystals (e.g. quartz, sapphire, calcite),muscovite mica, polymers (e.g. Polaroid polyvinyl alcohol PVA, DuPontpolyethylene terephthalate PET), or liquid crystal film. Thevoltage-controlled λ/2- and λ/4-3λ/4 phase retardation plates can bemade from nematic liquid crystal. Suitable fabrication techniques forsuch electro-optical components are described in U.S. Pat. No. 4,670,744to Buzak, and, U.S. Pat. No. 4,719,507 to Bos, both incorporated hereinby reference. Either a DC or square AC voltage with a frequency beyondthe response characteristics of the liquid crystal retarder can be used.Although high-speed phase-retarders with nanosecond response time arecommercially available, there is no apparent advantage to using suchdevices, due to the inherently slow nature of the mechanical trackingand focusing mechanisms used in the optical pick-up.

Referring to FIG. 17E, the photodetection unit 1706 of the type-1optical pick-up will now be described in greater detail. As shown, thephotodetection unit comprises: an array of detectors 170601, 170611 and170691, realized as an array of dichroic mirrors, for detecting,separating and directing along separate optical channels, the reflectedoptical signal components (i.e. reflected spectral components) of thereflected laser beam, characterized by wavelengths λ₁, λ_(i), λ_(N),respectively; and array of astigmatic lenses 170602, 170612, and 170692,each being disposed along an isolated optical channel, and focusing theindividual spectral component to a focal point; and an array ofmulti-functional (i.e. quandrant-type) photodetectors 170603, 170613 and170693, disposed at the focal points of astigmatic lenses 170602,170612, and 170692, respectively, for detecting the intensity of each ofthe reflected spectral components.

The operation of the type-1 optical pick-up (i.e. head) will bedescribed below. For purposes of illustration, it will be assumed thatinformation is stored within the multilayered optical storage media ofFIG. 11.

During an information reading operation, the media access controllerspecifies the wavelengths and polarization states of the spectralcomponents of the laser beam required to read information from aspecified information layer in a multi-layered/decked storage media ofthe present invention. In the illustrative embodiment of FIG. 17A,polarization of produced spectral components is achieved byelectronically controlling the voltages provided to voltage-controlledhalf-wave (λ/2) retardation plates 170105, 170115 and 170195 during theinformation reading process. For example, when no voltage (i.e. V_(i)=0)is applied to any particular half-wave (λ/2) retardation plate (e.g.170105, 170115 or 170195), the polarization-state of the spectralcomponent passing therethrough does not change from its originallyimparted polarization state. However, when a sufficient non-zero voltageV_(i) is applied to any particular voltage-controlled λ/2-plate, itchanges the polarization-state of the spectral component λ_(i) passingtherethrough from its originally imparted polarization state P₁ (e.g.from P₁ to P₂, or from LHCP-state to RHCP-state or vice versa). In thecase of optical storage media having linear polarization-selectiveproperties, each spectral component in the incident laser beam will havea linear polarization-state prespecified by the media access controller,and each information storage layer in the optical storage mediacorresponding will have a linear characteristic polarization-state. Insuch an illustrative embodiment, the function of each “activated”voltage-controlled half-wave (λ/2) retardation plate is to change thecharacteristic polarization-state of its input spectral component to thecharacteristic polarization state of its corresponding informationstorage layer from which prerecorded information is to be read.Consequently, the use of the λ/4-phase retardation plate is notrequired.

The polarization state of each spectral component in the composite laserbeam is selectively controlled in the above-described manner. After allof the specified spectral components have been simultaneously polarizedin accordance with media access controller specifications, the polarizedspectral components are combined into a single composite laser beam, andthen, in the case of a multi-decked system, the laser beam is focusedinto the information storage deck specified by the media accesscontroller. As the spectral component of the laser beam withcharacteristic wavelength and polarization state (λ_(i), P₂) fallsincident on the information storage layer-(λ_(i),P₁), it is transmitteddirectly therethrough; then when this spectral component (λ_(i), P₂)falls incident on the information storage layer-(λ_(i), P₂), it isintensity modulated thereby and reflects back towards the opticalreading head where the modulated spectral component is isolated from allother spectral components, detected and processed in a manner describedabove, to recover the bit stream (e.g. 11001111 . . . ) recorded in theaccessed layer.

Notably, during the information reading process, the focusing of eachincident spectral component (λ_(i), P₂) and the tracking of the same asit reflects off the (λ_(i), P₂) information storage layer can be carriedout using servo-control techniques. Advantageously, because the errorsignal for servo-control is derived from the shape of the laser beam,and not its polarization-state, laser beam focusing and tracking can becarried out in the present invention using highly-developed beamfocusing and tracking techniques used in conventional CD ROM drives tofocus and track linearly polarized beams. These techniques will bedescribed in detail hereinafter.

In the optical subsystem of FIG. 17A, only about half of the aperture ofthe objective focusing lens is used to focus the incident laser beamonto the optical storage medium, whereas the remaining portion of theaperture of the objective focusing lens is used to focus the reflectedlaser beam onto the detector. In many applications, it will be desiredto use the full aperture of the focusing objective lens in order tofocus both the incident and reflected laser beams during informationreading operations. Below will be described several ways in which toachieve a greater utilization of the aperture of the objective focusinglens.

As shown in FIGS. 17F and 17G, the aperture of the objective focusinglens is indicated by big solid circle, whereas the incident andreflected beams by dotted circles. While the light collection efficiencyof a lens is usually reduced gradually from its center, apertureutilization can be visualized (i.e. to a first-order approximation) asthe percentage area of the aperture that is covered. Thus, forarrangements with spatially separated beams, as illustrated in FIG.17F1, only (R₁/R₂)/2=(½)/2=25% of the aperture is used. While it isimpossible in practice to use 100% of the lens aperture, a “reentrant”or “retro-reflective” optical subsystem design will permit utilizationof about 85-98% of the objective lens aperture. As shown in FIG. 17F2,this is generally achieved by allowing the optical path of the reflectedlaser beam to retrace the optical path of the incident (i.e. incoming)laser beam. In order to achieve this in practice, a Faraday isolator isemployed along the overlapping optical path so that the aperture can becompletely used (i.e. 100% utilization). In this case, shown in FIG.17F2, the dotted circles representative of the incident and reflectedbeams are overlapped. If a small mirror is placed at the focused spot ofthe reflected beam, which has a size is only few microns, then the lightbeam can almost fill the aperture (i.e., approach 100% utilization ofthe aperture). Below, two retro-reflective techniques are applied in thedesign of optical reading pick-ups (of either type-1 or type-2) in orderto improve aperture utilization thereof.

As shown in FIG. 18, improvement in the aperture of the optical readinghead can be achieved by simply moving the beam-steering mirror to aposition between the laser source and beam collimating and circularizingoptics. As shown, the optical reading head of this alternativeembodiment comprises: a laser array 1801 for producing a laser beamhaving a characteristic wavelength; beam-shaping (i.e. collimating andcircularizing) optics 1802 for collimating and circularizing the laserbeam; a quarter-wave (λ/4) plate 1803 for imparting quarter-wave phaseretardation to the laser beam; a voltage-controlled half-wave plate 1804for imparting half-wave (λ/2) phase retardation to the laser beam, sothat it is imparted with the characteristic polarization-state specifiedby the media access controller; a first beam-steering mirror 1805, forsteering the polarized laser beam along the reading axis; aphase-grating 1806 for splitting (i.e. diffracting) the laser beam intothree different laser for use in tracking, as described hereinabove; afocusing objective lens 1807 for focusing the laser beam onto apreselected information storage layer in the optical storage mediahaving a corresponding characteristic wavelength and polarization-state,and thence for focusing the reflected laser beam back through plate 1804and 1803 and lens 1803; a first ultra-compact housing 1814 within whichphase-grating 1806 and objective lens 1807 are mounted (i.e. enclosed)and which is moveable up and down under servo-control for fine focusingof the incident laser beam during information reading operations; asecond ultra-compact housing 1815 with which beam-steering mirror 1805and ultra-compact housing 1814 are mounted and which is radiallytranslatable (i.e. moved) under servo-control for controlling thetracking of the incident laser beam during information storageoperations; a spherical aberration compensator (i.e. corrector) 1813, asdisclosed, for example, in FIGS. 11B and 11C; a second beam steeringmirror 1808 for steering the reflected laser beam along a lightdetection optical path; a relay lens 1809 disposed along the lightdetection optical path, for collimating the reflected laser beam; aspherical lens 1810 disposed along the light detection optical path, forfocusing the laser beam; a knife-edge 1811 disposed at the focal planeof a spherical lens 1810, near it focal spot; a dual-photodetectionarrangement 1812A centrally disposed along the light detection opticalpath, between dual-photodetectors 1812B and 1812C, for converting thereflected laser beam into two corresponding electrical analog signals Aand B (i.e. Data Signals), for (i) conditioning (e.g. summing A+B) andthereafter digitizing to produce a digital data signal, and (ii)detecting the “in-focus” condition of the incident laser beam duringinformation reading operations, in a manner which will be described indetail hereinafter; and a dual photodetection structure 1812B and 1812Cdisposed along the light detection optical path (i.e. created by abeam-splitter, not shown), and about dual-photodetector 1812A, forconverting the reflected laser beam into a corresponding electricalsignal, and detecting the “on-track” condition of the incident laserbeam during reading operations. In this alternative embodiment,polarization control is carried out upon the laser beam prior to itsincidence upon the optical storage media.

Yet another alternative embodiment for increasing the aperture of thefocusing objective lens of the optical reading head hereof isillustrated in FIG. 19. As shown, the retro-reflective optical readinghead of FIG. 19 comprises: a laser diode 1901 for producing a laser beamhaving a characteristic wavelength; beam-shaping (i.e. collimating andcircularizing) optics 1902 for collimating and circularizing the laserbeam; a polarizing beamsplitter 1903 for transmitting the incident laserbeam and reflecting the reflected laser beam; a Faraday optical isolator1904, for preventing laser light from reflecting back into the lasersource (i.e. restricting light to propagate in the forward direction); avoltage-controlled half-wave plate 1905 for imparting half-wave (λ/2)phase retardation to the laser beam, so that it is imparted with thecharacteristic polarization-state specified by the media accesscontroller; a first beam-steering mirror 1906, for steering thepolarized laser beam along the reading axis; a phase-grating 1907 forsplitting (i.e. diffracting) the laser beam into three different laserbeams for beam tracking, as described hereinbefore; a focusing objectivelens 1908 for focusing the laser beam onto a preselected informationstorage layer in the optical storage media having a correspondingcharacteristic wavelength and polarization-state, and thence forfocusing the reflected laser beam back through lens 1908 and phasegrating 1907 towards mirror 1906; a first ultra-compact housing 1912within which phase-grating 1907 and objective lens 1908 are mounted(i.e. enclosed) and which is moveable up and down under servo-controlfor fine focusing of the incident laser beam during information readingoperations; a second ultra-compact housing 1913 with which beam-steeringmirror 1906 and ultra-compact housing 1912 are mounted and which isradially translatable (i.e. moved) under servo-control for controllingthe tracking of the incident laser beam during information storageoperations; a spherical aberration compensator (i.e. corrector) 1911, asdisclosed, for example, in FIGS. 11B and 11C; an astigmatic lens 1909disposed along the light detection optical path, for focusing the laserbeam; and a quad-photodetection structure 1910A centrally disposed alongthe light detection optical path, between dual-photodetectors 1910B and1910C, for converting the reflected laser beam into four correspondingelectrical analog signals (i.e. Data Signals A, B, C and D) for (i)conditioning (e.g. summing A+B+C+D) and thereafter digitizing to producea digital data signal, and (ii) detecting the “in-focus” condition ofthe incident laser beam during information reading operations; and adual photodetection structure 1910B and 1910C disposed along the lightdetection optical path (i.e. created by a beam-splitter, not shown), andabout quad-photodetector 1910A, for converting the reflected laser beaminto a corresponding electrical signal, and detecting the “on-track”condition of the incident laser beam during reading operations. In thisalternative embodiment, polarization control is carried out upon thelaser beam prior to its incidence upon the optical storage media. Whilethis retro-reflective optical pick-up design is preferred, the presentcost of Faraday optical isolators may make this technique less practicalin many applications.

In FIG. 20A, the type-2 optical pickup (i.e. reading head) is shown. Themajor difference between the type-2 system and the type 1 system is thatfor the type-2 system the polarization of the reflected laser signal,rather than that of the incident laser beam, is controlled by the mediaaccess controller. In this system, it is assumed that incident laserbeam has both P₁ and P₂ components. For circular polarization-sensitivemedia, (e.g., CLC-based optical media), the P1 and P2 components of theincident laser beam may be either linear polarized or unpolarized. Ineither case, the incident laser beam is composed of two (i.e. left andright handed) circularly polarized components. In the circularpolarization sensitive media case, the direction of linear polarizationis not important. For linear polarization-sensitive media, the P₁ and P₂components of the incident laser beam may linear or unpolarized, withthe understanding that linear polarization is at 45° with respect to thex or y axis. In the linear polarization sensitive case, the twocomponents P₁ and P₂ are equal in amplitude, thereby ensuring similarsignal-to-noise ratio (SNR) for both P₁ and P₂ channels in this opticalpick-up system. This requirement may be relaxed in the sense that aslong as SNR is adequate, the non-zero (instead of 45°) inclination isnot strictly required.

As shown in FIG. 20A, the type-2 optical pick-up subsystem comprises anassembly of components, namely: a laser beam producing unit 2001 forproducing a collimated and circularized laser beam having one or morespectral components (i.e. lines) with characteristic wavelengths, eitherlinearly polarized or unpolarized, specified by the media accesscontroller; a first beam-steering mirror 2002, for steering the producedlaser beam after it has been collimated and circularized; aphase-grating 2003 for producing three laser beams for use in beamtracking; a focusing objective lens 2004 for focusing the collimatedlaser beam as it is directed incident to the optical storage mediahereof; a first ultra-compact housing 2009 within which phase-grating2003 and objective lens 2004 are mounted (i.e. enclosed) and which ismoveable up and down under servo-control for fine focusing of theincident laser beam during information reading operations; a secondultra-compact housing 2009 with which beam-steering mirror 2002 andultra-compact housing 2008 are mounted and which is radiallytranslatable (i.e. moved) under servo-control for controlling thetracking of the incident laser beam during information storageoperations; a spherical aberration compensator (i.e. corrector) 2007, asdisclosed, for example, in FIGS. 11B and 11C; a second beam steeringmirror for steering the produced laser beam after it has been reflectedfrom the information storage medium and focused through the objectivefocusing lens; and an array of photodetectors 2006 for detecting thespectral components of the composite laser beam as they reflect offcorresponding information storage layers in the optical storage mediumduring information reading operations.

In FIG. 20B, the subcomponents of the laser beam producing unit areshown in greater detail. As shown, the laser beam producing unit 2001comprises: an array of lasers, e.g., lasers 200101, 200111 and 200191,for producing spectral components with characteristic output wavelengthsλ₁, λ_(i), and λ_(N), respectively, and a predetermined polarizationstate (e.g., linear polarization state); an array of collimating lenses,200102, 200112, and 200192, for collimating the spectral output oflasers 200101, 200111 and 200191, respectively; an array of anamorphicprisms, 200103, 200113 and 200193, for circularizing the spectral outputfrom collimating lenses, 200102, 200112, and 200192, respectively; andan array of dichroic mirrors 200104, 200114 and 200194, respectively,for combining the polarized spectral components into a single compositelaser beam precharacterized for reading particular information storagelayers having characteristic wavelengths and polarization-states whichcorrespond to the wavelengths and polarization-states of the spectralcomponents of the incident composite laser beam. As the incident laserbeam has two polarization components for type 2 optical pick-up, thepolarization-state of the optical storage media is not important. Thedrawback of this system, however, is that unless both the P₁ and P₂information storage layers (corresponding to characteristic wavelengthλ_(i)) are being accessed, half of the incident laser beam is not used(i.e. it is wasted).

Referring to FIG. 20C, the photodetection unit 1706 of the type-2optical pick-up will now be described in greater detail. As shown, thephotodetection unit comprises: an array of dichroic mirrors 200601,200611 and 200691, for separating and directing the spectral componentsof the reflected laser beam, characterized by wavelengths λ₁, λ_(i), andλ_(N), respectively, along a plurality of spatially isolated opticalchannels; an array of voltage-controlled, circular-polarizationselective shutters for light transmission regulation, comprising anarray of voltage-controlled λ/2-phase retarders 200602, 200612 and200692 and an array of circular polarizer filters 200603, 200613 and200693, respectively; an array of astigmatic lenses 200604, 200614 and200694, each disposed along an isolated optical channel, and focusingthe individual spectral component to a focal point; and multi-functionphotodetector 200605, 200615 and 200695, disposed at the focal points ofastigmatic lenses for detecting the intensity of each of the reflectedspectral components.

The operation of the type-2 optical pick-up (i.e., head) will bedescribed below with reference to the multi-layered (CLC-based) opticalstorage medium of FIG. 13.

Central to understanding the operation of the type-2 optical pickupdescribed above, is recognizing that a linearly polarized laser beamhaving characteristic wavelength λ_(i) consists fundamentally of both aRHCP spectral component and a LHCP spectral component, each having thesame characteristic wavelength λ_(i) and an equal amplitude. Thisfundamental fact of physics is well known in the art and is discussed atgreat length in, for example, “Principles of Optics” by M. Born and E.Wolf, published by Pergamon Press, New York, and “Geometrical andPhysical Optics” by R. S. Longhurst, published by Longman, New York. Inessence, the linearly polarized laser beam can be represented as:

a _(x) sin(ωt)=½{a _(x) sin(ωt)+a _(y) cos(ωt)}+½{a _(x) sin(ωt)−a _(y)cos(ωt)}

where a_(x) and a_(y) are unit polarization vectors along x and y axes,respectively, and the terms in { } represent circular polarizationvectors of magnitude equal to the square root of 2. More rigorously, itcan be shown that the polarization vectors P1 and P2 (i.e. where 1 and 2are either x and y, or RHCP and LHCP) form a complete set ofeigenvectors, in terms of which any vector can be expressed.

As the linearly polarized laser beam reflects off each particular“prerecorded” information storage cell in a particular paired P₁-P₂layer of the CLC-based storage medium, its spectral composition istypically transformed. The exact spectral composition of the linearlypolarized laser beam after it is reflected from a particular informationstorage cell will depend on the logical states of the information bitsrecorded at that particular storage cell of the paired P₁-P₂ informationstorage layer. When considering the information storage medium shown inFIG. 20A, four rules are adequate to describe the behavior of incidentlaser beam used during information reading operations. These rules ofbehavior are set forth below.

When the laser beam is incident upon the information storage cell atposition (a) where a logical “1” has been recorded on the P₁-polarized(i.e. RHCP) layer and a logical “0” has been recorded on theP₂-polarized (i.e. LHCP) layer, then only the P₁-polarized component ofthe linearly polarized incident laser beam will reflect off theinformation storage cell at that location. When the laser beam isincident upon the information storage cell at position (b) where alogical “1” is recorded on both the P₁-polarized layer and theP₂-polarized layer, then both P₁ and P₂ polarized components of theincident laser beam will reflect off the information storage cell atthat location. When the laser beam is incident upon the informationstorage cell at position (c) where a logical “0” is recorded on theP₁-polarized layer and a logical “1” is recorded on the P₂-polarizedlayer, then only the P₂-polarized component will reflect off theinformation storage cell at that location. When the laser beam isincident upon the information storage cell at position (d), whereneither a logical “0” is recorded on both the P₁-polarized layer and theP₂-polarized layer, then neither the P₁ polarized component or the P₂polarized component of the incident laser beam will reflect off theinformation storage cell at that location, but rather both of thesespectral components will be transmitted through the informationrecording medium at this cell location.

In order to detect the polarization state of each of the spectralcomponents of the reflected laser beam, and thus detect the informationbit recorded at each information storage cell within a particular trackof the optical storage medium, the voltage-controlled,circular-polarization selective shutter is employed for eachcharacteristic wavelength λ_(i) in order to control the transmission ofreflected laser light towards the photodetector for detection. Asmentioned above, each voltage-controlled shutter consists of avoltage-controlled λ/2-phase retardation plate (e.g. 200612) and acircular (e.g. RH) polarizer (e.g. 200613). When the media accesscontroller determines that the (λ_(i), P₁)-layer is to be read, novoltage is applied to the voltage-controlled phase plate, and thecircular-polarization selective shutter transmits only the P₁ (i.e.RHCP) polarized spectral component of the reflected laser beam. In suchinstances, the photodetector receives only the reflected spectralcomponent reflected from the (λ_(i), P₁)-layer as the P₂ (i.e. LHCP)polarized spectral component from the (λ_(i), P₂)-layer is blocked bythe circular-polarization selective shutter. The intensity of thedetected spectral component carries information corresponding to thelogical value recorded at that information cell location. Tracking andfocusing onto the (λ_(i), P₁)-information layer is servo-controlledusing the P₁-polarized spectral component of the reflected laser beam.

When the media access controller determines that the (λ_(i), P₂)information storage layer is to be read, then V_(i) is applied to thevoltage-controlled λ/2-plate and the circular-polarization selectiveshutter transmits only the P₂-polarized spectral component of thereflected laser beam. In this case, the detector receives only thereflected spectral component from the (λ_(i), P₂)-layer as theP₁-polarized spectral component from top (λ_(i), P₁)-layer is blocked bythe circular-polarization selective shutter. The intensity of thedetected spectral component carries information corresponding to thelogical value recorded at that information cell location. Tracking andfocusing of this bottom (λ_(i), P₂)-information layer isservo-controlled using the P₂-polarized spectral component of thereflected laser beam.

Notably, when using a type-2 optical pickup, the techniques shown inFIGS. 18 and 19 can also be used to improve the aperture of the beamfocusing objective lens. Also, when using linear polarization-selectivemedia, rather than circular-polarization selective media as describedabove, the polarization of the incident laser beam will preferablyaligned at 45° with respect to the x-axis. In such an alternativeembodiment, a linear polarizer will be used in lieu of each circularpolarizer (i.e. 200692, 200612, 200602) in order to produce avoltage-controlled linear-polarization polarization selective shutter.In a manner similar to the circular-polarization selective shutterdescribed above, this linear-polarization selective shutter willtransmit to the photodetector only reflected spectral components havingboth the characteristic wavelength and linear-polarization-state of theinformation storage layer specified to be read by the media accesscontroller by way of the voltage control signals provided to thelinear-polarization selective shutter.

In general, there are several schemes for maintaining the incident laserbeam in the “in-focus” condition during reading operations, namely the“knife-edge” technique illustrated in FIGS. 18, 18A, 18B and 18C; andthe “astigmatic difference” approach. These two techniques will bedetailed below.

As shown in FIGS. 18, 18A, 18B and 18C, the knife edge techniqueemployed in the laser beam detection subsystem of FIG. 18 exploits thefact that the divergence of the reflected beam changes as the opticalstorage media moves into its “out of focus” condition. In the opticalarrangement shown in the FIG. 18, the laser beam passes through the beamshaper 1802 twice, so that its final shape is elliptical when theincident laser beam is in its “out of focus” condition, that is, whenthe analog Data Signals A and B produced from the light detector 1812Aand the Focus Error Signal (FES) derived therefrom satisfy either of thefollowing conditions: FES=A−B>0 or FES=A−B<0. When both halves of thelight detector 1812A are evenly illuminated, then the “in-focus”condition is obtained (i.e. FES=A−B=0). As the optical storage media ismoved “out of focus”, the reflected laser beam is divergent ofconvergent and the focal image moves causing the knife edge 1811 tounevenly illuminate the dual detector 1812A, resulting in a non-zeroerror signal, which is used in autofocusing lens 1908 underservo-control. Notably, when using the Foucault knife edge method ofFIGS. 18A, 18B and 18C, the reflected laser beam need not be circular.

The astigmatic method, employed in the illustrative embodiment of FIG.19, is preferred over the “knife edge” by virtue of its simplicity andeasy alignment. In the system of FIG. 19, quadrant detector 1910A isplaced along each optical channel, with its axes at 45° with respect tothe paper, in the middle between the two focal planes at the circle ofleast confusion, which represents in-focus condition as the returninglight beam is collimated by the objective lens (e.g. 1908). Theastigmatic lens assembly 1909 has an axial focal difference of less thanone millimeter. When the optical storage layer (e.g. the optical storagedisc) moves away from the in-focus condition, then the laser beam eitherconverges or diverges and the light distribution of the reflected laserbeam at the quadrant detector 1910A becomes elliptical. By summing theData Signals A and C and B and D from opposing detector segments toproduce first and second sums (A+C) and (B+D), and subtracting the firstsum from the second sum, a Focusing Error Signal (FES) is automaticallygenerated (i.e. FES=(A+C)−(B+D)) for use in controlling the focus of theincident laser beam under servo-control. When the optical storage discmoves away from its in-focus condition, the returning laser beam iseither converging or diverging and thus elliptical in beam cross-sectionat the quadrant photodetector, thereby generating a non-zero focus errorsignal FES=(A+C)−(B+D). In the in-focus condition, the reflected laserbeam is collimated and therefore the quadrant detector generates a zeroerror signal. Notably, the produced error signal is positive when theoptical storage medium moves away from the objective lens, whereas theerror signal is negative when the optical storage medium moves closertowards the objective lens. The produced error signal is then used bythe servo-mechanism to control direction in which the objective lens ismoved in order maintain the in-focus condition. However, as will becomeapparent hereinbelow, the sign of the error signal will typically dependon the particular embodiment employed in carrying out auto-focusingcontrol. Notably, when using the astigmatic technique of FIG. 19, thereflected laser beam must have a circular beam shape. Theabove-described auto-focusing technique can be easily realized in anyembodiment of the present invention. Greater details on automaticfocusing mechanisms for use in the optical storage systems hereof can befound in ‘The Physical Principles of Magneto-Optical Recording’ by M.Mansuripur, published by Cambridge University Press, 1995.

In each of the illustrative embodiment of the present invention, it isnecessary to automatically and precisely control incident laser beamduring information reading operations so that it accurately follows theinformation track which the optical pick-up is instructed to read underthe control of the system controller. In FIG. 19D, the process ofauto-tracking using servo-control is schematically illustrated. Forpurposes of illustration, the system of FIG. 19 will be used to describethis process, noting that any one of the other illustrative embodimentspossesses this system functionality.

In FIGS. 19E, 19F and 19G, the dual photodetection cells 1910B and 1910Care shown closely positioned on opposite sides of the quadrantphotodetector 1910A. In a multi-wavelength system, each isolated opticalchannel within the photo-detection subsystem will have a similarphoto-detection arrangement for use in laser beam focusing and trackingcontrol functions. In practice, all six of these photodetective surfacescan be realized on a single integrated circuit using state of the artmicrofabrication techniques known in the art.

As shown in FIGS. 19E, 19F adn 19G, the photonic energy of the centerbeam having characteristic wavelength λ_(i) and associated with the 0-thdiffraction order (i.e. labeled 1) is focused onto the quadrantphotodetector 1910A, and is used to (i) read the information recorded onthe optical storage layer and (ii) carry out the above-describedautofocusing function under servo-control. The photonic energy of thetwo weaker beams associated with the +1st and −1st diffractive orders(i.e. labeled 2 and 3) are used for automatic laser beam tracking underservo-control. The manner in which this latter function is carried outin practice is as follows. When the incident laser beam is at the centerof the information storage tack, as shown in FIG. 19D, then thephotodetectors E and F (i.e. 1910B and 1910C, respectively) are evenlyilluminated by the reflection of beams 2 and 3. Thus, the value of theTrack Error Signal (TES)=E−F=0. However, when the incident laser beamdeviates from the center of the information storage track, then one ofthe tracking beams 2 or 3 is reflected back to the photodetector withrelatively stronger intensity because a larger reflective area on thetrack has been illuminated. Under such conditions, the electricalsignals produced from photodetectors E and F are not equal, and a signednon-zero error signal is generated for use by an actuator to move theoptical pickup sideways in an appropriate manner.

In the above-described illustrative embodiments, laser diodes are usedas light sources for reading information stored in the optical storagemedia of the present invention. Because of the unique reflectionproperties of information storage layers hereof (i.e. particularly theCLC layers), the reflected laser beam has the same circular polarizationas that of the incident laser beam. Consequently, there is an advantagein using a non-retroreflective optical pick-up design in order toeliminate the intensity of noise produced as a result of to opticalfeedback with the laser. In order to prevent optical feedback with thelaser, it is preferred to use the optical pick-ups shown in FIGS. 17A,19, 21 and 22 when reading information from a CLC-based informationstorage disk. If a retroreflective design must be used, thenlow-coherence lasers (e.g. self-pulsating lasers) are preferred for usein the optical pick-up of the present invention.

In general, when it is desired that only one information track on onlyone layer of a P₁-P₂ information storage structure is to be read out atone instant in time (i.e. serially), then there is no requirement thatthe information tracks on these information layers are preciselyaligned, or that they otherwise overlay each other. This informationaccess (i.e. reading method) method allows the tracking and focusing onthe particular storage layer being read. A primary advantage associatedwith this technique is that the information storage (i.e. recording)tracks of both the upper and lower layers of the P₁-P₂ informationstorage structure need not be aligned, thereby improving greatly themanufacturability of such information storage devices. Consequently,such embodiments of the information storage device of the presentinvention can be made by simply laminating two layers with a crudeoverlaying technique or none at all.

However, when it is desired to simultaneously read information from boththe top and bottom layers of a P₁-P₂ information storage structure, itis then necessary that the information storage tracks thereon beprecisely aligned with each other. In addition, a suitably adaptedoptical reading pick-up is required to carry out this simultaneousinformation access method. In FIG. 21, a schematic diagram for such anoptical pick-up is shown. Advantageously, this optical pickup can readinformation from both the top and bottom layers of a P₁-P₂ informationstorage structure, thereby doubling (or multiplying) readout throughout.

As shown in FIG. 21, the optical pickup for parallel reading comprises anumber of subcomponents, namely: a laser beam producing unit 2101 forproducing a collimated and circularized laser beam having two spectralcomponents (i.e. lines) each having the same characteristic wavelengthand orthogonally-different polarization-states P₁ and P₂, respectively,specified by the media access controller; astigmatic lenses and ananamorphic prism 2102 for collimating and circularizing the compositelaser beam; a first beam-steering mirror 2103, for steering thecomposite laser beam after it has been collimated and circularized; aphase-grating 2104 for splitting the composite laser beam into threespots for tracking and data retrieval; a focusing objective lens 2105with a sufficient depth of focus for focusing the collimated laser beamonto the optical storage media hereof; a first ultra-compact housing2113 within which phase-grating 2104 and objective lens 2105 are mounted(i.e. enclosed) and which is moveable up and down under servo-controlfor fine focusing of the incident laser beam during information readingoperations; a second ultra-compact housing 2114 with which beam-steeringmirror 2103 and ultra-compact housing 2113 are mounted and which isradially translatable (i.e. moved) under servo-control for controllingthe tracking of the incident laser beam during information storageoperations; a spherical aberration compensator (i.e. corrector) 2112, asdisclosed, for example, in FIGS. 11B and 11C; a second beam-steeringmirror 2106 for steering the reflected laser beam along a second opticalpath; a dichroic mirror for splitting the reflected laser beam intopolarized spectral components P₁ and P₂; a first astigmatic lens 2108for focusing the reflected laser beam containing polarized spectralcomponent P₁; a photodetector 2109 for detecting the polarized spectralcomponent P₁ reflected laser from the corresponding information storagelayer (λ_(i), P₁) in the optical storage media; a second astigmatic lens2110 for focusing the reflected laser beam containing polarized spectralcomponent P₂; a photodetector 2111 for detecting the polarized spectralcomponent P₂ reflected laser from the corresponding information storagelayer (λ_(i), P₂) in the optical storage media.

The optical subsystem of FIG. 21 can be readily adapted for realizationas either a type-1 pick-up design shown in FIG. 17A, or a type-2 pick-updesign shown in FIG. 20A. When desiring to build a type-1 opticalpick-up for simultaneously reading paired P₁-P₂ layers, the laser beamproducing and spectral component detection units of FIGS. 17B and 17Ewill be included in the optical pick-up of FIG. 21. When desiring tobuild a type-2 optical pick-up for simultaneously reading paired P₁-P₂layers, the laser beam producing and spectral component detection unitsof FIGS. 20B and 20C will be included in the optical pick-up of FIG. 21.In order to improve the aperture of the objective focusing lens of theoptical pick-up of FIG. 21, the optical techniques disclosed in FIGS. 18and 19 can be used as taught hereinabove. The depth-of-focus adjustmentmechanisms shown in FIGS. 11 and 12 can be used of the optical pick-upof FIG. 21. Optionally, polarization band-pass filters can be placed infront the photodetectors. It is clear to those skilled that, for linearpolarization-selective storage media, the circular polarizationbeamsplitter 2107 is replaced by a linear polarization beam splitter toseparate the two orthogonally polarized signals.

Any of the embodiments of the optical pick-up subsystem of FIG. 21described above can be used to simultaneously read informationprerecorded on the tracks of paired P₁-P₂ layers in an informationstorage structure hereof. However, to use this optical pick-up andthereby double the information readout rate of the system, theinformation tracks formed in these paired layers must be preciselyaligned. Notably, this places strict requirements on the manufacturingprocess.

There are a number of techniques for manufacturing information storagedevices having precisely aligned information tracks formed on eachpaired set of P₁-P₂ layers. The precision alignment techniques used inthe photolithographic manufacturing of integrated circuit (IC) chips maybe used in the alignment of information tracks recorded on paired P₁-P₂layers. However, these techniques may not be practical at present forlow-cost mass production of CD-ROM disks according to the presentapplication. For optically-based Write-Once and Read-Many-times (WORM)disks, the information tracks recorded on the paired layers can beautomatically aligned during writing operations. Thus, type 2 opticalpickup are preferred for WORM drives.

For backward compatibility with conventional information storage andretrieval systems (e.g. CD ROM drives), the information storage andretrieval system of the present invention, such as the CD drive of FIGS.3A and 3B, includes a mechanism for automatically determining whether aconventional, multi-decked or multi-layered CLC disk has been insertedinto its support tray for reading. In FIG. 23, an automatic disk-typesensing program is shown for use with the system of the presentinvention. In a manner similar to that of conventional CD drives, theCLC CD drive of the present invention scans (i.e. reads) the tracks fromnear the center of the disk to the outer rim thereof. Preferably, acoded message of a few bytes is pre-recorded on the first track of CLCdiscs of the present invention. When this coded message is scanned (i.e.read), the automatic disk-type sensing circuit in the CD drive unithereof to read information about the inserted disk, an determines thedisk type (e.g. two-layers, multilayers or multi-decked). If no suchcoded message is read, then the disk is automatically recognized as aconventional CD by the CD drive unit of the present invention.Alternatively, it would be desired to have conventional CD-ROMs coded,as this would allow them to be positively recognized. Thus by simplyreading the information track of the disc, the system can properly setup the necessary servo-control to read the optical disc.

The two basic types of optical pickup designs described in great detailabove are backward compatible, in that they can also be used to readregular CD-ROMs, as well as CD ROMs constructed in accordance with theprinciples of the present invention. When the optical pickups of theillustrative embodiments recognize that a conventional CD ROM has beeninserted into its support tray for reading purposes, then automaticallythe electro-optics thereof are controlled in a particular manner inorder to be capable of reading the conventional CD ROM, or the opticalstorage disc hereof, as the case may be. The operation of theseillustrative embodiments will be described below when reading aconventional CD disc.

For example, when the type-1 optical pick-up of FIG. 17 recognizes thata conventional CD has been inserted within its support tray and isattempting to be read by a user, then the control voltage to thevoltage-controlled phase-retardation plate does not matter, as theconventional CD ROM disc is not polarization selective. The reflectedlaser beam from the conventional CD disc is used for servo-controllingfocusing and tracking.

When the type-1 optical pick-up of FIG. 17 recognized that a CLC CD ofthe present invention has been inserted within its support tray and isattempting to be read by a user, then the control voltage to thevoltage-controlled phase-retardation plate is automatically set to zero,causing the laser beam incident on the CLC disc to be circularlypolarized (e.g. RHCP or LHCP) prior to incidence on the CLC disc. Uponreflection from metallic surface of the conventional CD disc, thecircular polarization direction of the reflected laser beam changes(e.g. RHCP to LHCP). The reflected laser beam from the CD disc is usedfor servo-controlling focusing and tracking.

When the type-1 optical pick-up of FIG. 19 recognized that aconventional CD has been inserted within its support tray and isattempting to be read by a user, then the control voltage to thevoltage-controlled phase-retardation plate does not matter, as itchanges P1 to P2 or vice versa. The reflected laser beam from the CDdisc is used for servo-controlling focusing and tracking.

When the type-1 optical pick-up of FIG. 19 recognizes that a CLC CD ofthe present invention has been inserted within its support tray and isattempting to be read by a user, then the control voltage to thevoltage-controlled phase-retardation plate is automatically set to zero,causing the laser beam incident on the CLC disc to be circularlypolarized (e.g. RHCP or LHCP) prior to incidence on the CLC disc. Uponreflection from metallic surface of the conventional CD disc, thecircular polarization direction of the reflected laser beam changes(e.g. RHCP to LHCP). The reflected laser beam from the CD disc is usedfor servo-controlling focusing and tracking.

When the type-2 optical pick-up of FIG. 21 recognizes that aconventional CD has been inserted within its support tray and isattempting to be read by a user, then the control voltage to theelectrooptic phase-retarder in front of the photodetector does notmatter, as the conventional CD is not polarization selective. Thereflected laser beam from the conventional CD disc is used forservo-controlling focusing and tracking.

When the type-2 optical pick-up of FIG. 21 recognizes that a CLC CD hasbeen inserted within its support tray and is attempting to be read by auser, then the control voltage to the electrooptic phase-retarder infront of the photodetector is automatically set to V_(λ/4), causing thepolarization state of the circular polarization shutter to be eitherRHCP or LHCP. Then when the linearly polarized incident laser beamreflects off the metallic surface of the CLC CD, it will automaticallybe circularly polarized and thus transmitted without any attenuationthrough the circular polarization shutter located before thephotodetector.

In FIG. 22, an information reading system is shown having aretro-reflective optical reading head similar to that shown in FIG. 19,in addition to employing disc recognition logic. The system comprises: alaser diode 2201 for producing a laser beam having a characteristicwavelength; beam-shaping (i.e. collimating and circularizing) optics2202 for collimating and circularizing the laser beam; a polarizingbeamsplitter 2203 for transmitting the incident laser beam andreflecting the reflected laser beam; a Faraday optical isolator 2204A,for preventing laser light from reflecting back into the laser sourceand a voltage-controlled half-wave plate 2204B for imparting half-wave(λ/2) phase retardation to the laser beam, so that it is imparted withthe characteristic polarization-state specified by the media accesscontroller; a beam-steering mirror 2205, for steering the polarizedlaser beam along the reading axis; a phase-grating 2206 for splitting(i.e. diffracting) the laser beam into three different laser beams forbeam tracking, as described hereinbefore; a focusing objective lens 2207for focusing the laser beam onto a preselected information storage layerin the optical storage media having a corresponding characteristicwavelength and polarization-state, and thence for focusing the reflectedlaser beam back through lens 2207 and phase grating 2206 towards mirror2205; a first ultra-compact housing 2210 within which phase-grating 2206and objective lens 2207 are mounted (i.e. enclosed) and which ismoveable up and down under servo-control for fine focusing of theincident laser beam during information reading operations; a secondultra-compact housing 2211 with which beam-steering mirror 2205 andultra-compact housing 2210 are mounted and which is radiallytranslatable (i.e. moved) under servo-control for controlling thetracking of the incident laser beam during information storageoperations; a spherical aberration compensator (i.e. corrector) 2212, asdisclosed, for example, in FIGS. 11B and 11C; an astimatic lens 2208disposed along the light detection optical path, for focusing the laserbeam; multi-function photodetector 2209 disposed along the lightdetection optical path, for converting the reflected laser beam into acorresponding electrical signal for signal processing; and discrecognition logic 2213, illustrated in greater detail in FIG. 23. Inthis alternative embodiment, polarization control is carried out uponthe laser beam prior to its incidence upon the optical storage media.While this retro-reflective optical pick-up design is preferred, thepresent cost of Faraday optical isolators may make this technique lesspractical in many applications.

In general, the process for manufacturing CLC disks is different fromthe process employed in the manufacture of conventional CD ROMs.Conventional CD-ROM disks usually employ the EFM code which translatesan 8-bit data byte into a 14 optical bit pattern in which no fewer than2 ‘0’s and more than 10 ‘0’ may occur between any two consecutivelyoccurring ‘1’s. In the CLC CD ROMS of the present invention, a datacode, such as the so-called 8-14 modulation (EFM) code may be used.However, it is understood that other code may be used to fully exploitfor optical storage media of the present invention.

Below several methods are disclosed in detail for producing multilayerCLC storage media.

When sequentially reading information tracks on CLC layers, it is notessential that the information tracks recorded on paired layers bealigned. This is because focusing and tracking are servo-controlled withrespect to the track on a particular CLC layer being read. Such diskscan be made by laminating information storage layers realized aspre-recorded disks of very thin thickness.

In order to greatly reduce the investment cost of research anddevelopment in equipment for manufacturing the optical media of thepresent invention, the precision molding process used to manufactureconventional CD-ROMs can be modified and thereafter used to practice thepresent invention. As shown in FIGS. 24A to 24F, the method of thepresent invention comprises the using special machinery to make a mastermold for CLC CDs, from which many duplicate stamping molds can be madefor mass production of CLC CDs.

As shown in FIG. 24A, the first step of the method involves applying a“positive” photoresist onto a flat substrate having a spin coatingtechnique. Preferably, the photoresist has a low-thermal expansioncoefficient. In order to make a matched pair of molds for stampingprecisely aligned disks, the substrate must be made from an opticallytransparent material, such as glass or like material. In someapplications, a negative photoresist can be used, although the followingprocessing steps will require modification in a manner readily apparentto those skilled in the art.

As shown in FIG. 24B, the second step of the method involves using afocused laser beam in order to directly expose the photoresist layeralong a continuous spiral track, at either a constant linear velocity(CLV) or constant angular velocity. Exposing the photoresist layer atconstant linear velocity provides higher recording capacity overconstant angular velocity (CAV) or zoned constant angular velocity(ZCAV) format which requires concentric information storage tracks. Insome applications, the CAV and ZCAV formats may be preferred becausethese offer a faster access time than the CLV format. The exposure ofthe photoresist by the writing laser beam is controlled by digitallymodulating the laser beam using an encoded digital signal which iseither feed sequentially or in reverse order. This method of exposure isusually called direct-write, as no mask is used. To make preciselyoverlaid (i.e. aligned) tracks on both sides of the substrate, thespiral or concentric tracks must be exactly written on the matched pairof molds. This can be achieved by precisely controlling the rectilinear(x,y) or polar (r,θ) coordinates of the writing laser beam. The (x,y)-or (r,θ)-position is servo-controlled by laser interferometers whichhave an accuracy within 5 nanometers, which is more than adequate forthis use. The θ-position can be precisely controlled by an angular (i.e.optical or magnetic) encoder mounted on the spinning shaft supportingthe substrate disc. The accuracy of this angular encoder should bebetter than milli-arcseconds. As shown in FIG. 24C, the exposedphotoresist is then developed using a wet chemical etching agent or adry plasma etching agent, thereby leaving intact on the substratesurface, the area of the substrate coated with photoresist. Thisremaining area corresponds to logical ‘1’ values.

In an alternative embodiment of the present invention, photoetchingsteps (B) and (C) above can be replaced by a single photoablativeprocess carried out using an excimer laser which directly etches adigital data pattern into the surface of the disc substrate, using alaser beam having ultraviolet spectral components that photochemicallydecompose organic solid, such as plastic, into gaseous products.

As shown in FIG. 24D, the next step of the method involves evaporating athin metal film onto the etched away surface of the disc substrate. Asshown in FIG. 24E, the method involves using a lift process in order towash away the unexposed photoresist with metal film on top, leaving ametal thin film dot pattern, corresponding to digital data patternencoded in to the surface of the disc substrate. At this stage of themethod, the digital information pattern recorded on the CD master moldcan be read with high information fidelity using a special CD drive. Therepairs, so-called opens, ‘pin-holes’ and ‘mousebites’, shorts and otherdefects, can be made using laser deposition or deletion, if repairable.As shown in FIG. 24E, the method then involves using either vacuumdeposition or electrode-less plating in order to increase the hardnessof the protrusion pattern on the surface of the master mold.

After the master mold has been made, many duplicate stamping molds canbe made for mass production of CLC-type CDs. This involves using astamping mold duplicating process which essentially uses the master moldto press several so-called daughter molds of opposite image. Thedaughter molds are then plated to increase the surface hardness forstamping operations. This technique is commonly used in the productionof conventional CD ROMs. The resulting stamping mold has protrusionswhere logical “1”s are recorded, and pits where logical “0”s arerecorded.

Having made a duplicate stamping mold using the method described above,a CLC-based information recording layer is produced in the followingmanner. With the stamping mold installed in a conventional stampingmachine, the stamping mold halves are heated. Then a glob of plastic isinjected into the stamping mold and subsequently pressed. After cooling,the stamping machine ejects the pressed disk which has pits of onemicron or deeper where logical “1”s are recorded. These pits are thenfilled with an appropriate CLC material having a negative dielectricanisotropy. Preferred CLCs are polymers having a high molecular weight,and a solid state over the operating temperature range. One suitableclass of cholesteric liquid crystals is called the polymeric liquidcrystals, which include polysiloxanes, di-acrylates, di-epoxides,di-vinylethers and others, that are essentially an anisotropic glassthat exhibits the physical properties of a normal liquid crystal. Othersuitable class of cholesteric liquid crystals is called nematic liquidcrystals, which may include chiral additives. Regardless of theparticular CLC material used, the resulting structure is a CLC-basedinformation storage layer having information recording tracks withstorage cells digital information states (i.e. values) encoded into thesurface structure of the layer.

In FIGS. 25A to 25D, a method is disclosed for making a double-sided CLCdisc using two separate stamping operations. In general, the first stepof the method involves producing, as described above, separate stampingmolds, from which first and second storage (i.e. recording) layers (e.g.discs) of the paired CLC information storage structure (e.g.double-layered CD disc) can be made using the general pressing andfilling method described above. These information storage layers arelabeled as the A-side and the B-side, respectively. Then as shown inFIG. 25B, the pits on the stamped disks are filled with an appropriateCLC having a negative dielectric anisotropy. Then as shown in FIG. 25C,both sides of the CLC-filled discs are capped with a transparent thinplastic film. If simultaneous readout is to be employed, then theplastic film must be sufficiently thin in order to accommodate the depthof focus of the focusing objective lens of the optical pick-up.Sub-micron film thicknesses with surface consistency and uniformity canbe obtained by spinning techniques well known in the art. As shown inFIG. 25D, the filled layer is first heated above the glass temperatureof the liquid crystal material, and then subjected to an externalelectric field applied normal to the layer in order to align the CLCcells into the planar state. After the aligning operation, the layer iscooled. Notably, U.S. Pat. No. 5,353,247 to co-applicant, incorporatedherein by reference, teaches other techniques for aligning CLCinformation cells in order to write a logical ‘1’ information state.Then, the information storage disks containing recorded informationcells are laminated or bonded together, as disclosed in FIG. 25E, toform a paired CLC information storage disk. As shown in FIG. 25F, thepaired CLC disc can be laminated to a plastics substrate for improvingthe structural integrity of the resulting CLC compact disc.

Referring to FIGS. 26A to 26D, a method will now be disclosed for makinga double-sided CLC disc in a single stamping operation. The first stepof the method involves producing a stamping system comprising both a topmolding surface and a bottom molding surface, each having pit (i.e.incised) patterns formed therein which correspond to the digitalinformation patterns to be recorded in the resulting paired informationstorage structure. Using this molding system, the pit patternscorresponding to the digital information patterns to be recorded in theresulting paired information storage structure, are simultaneouslypressed into the opposite sides of a plastic disc in a single stampingoperation. The resulting pressed disc is shown in FIG. 26A. Then asshown in FIG. 26B, the pits on each side of the pressed disc are filledwith appropriate CLC material. Then as shown in FIG. 26C, the topsurface of the filled disc is coated with a thin plastic film, whereasthe lower side thereof is capped with a thicker plastic film,functioning as a substrate. Thereafter, digital information value arerecorded on the information storage cells on both sides of the disc byaligning the CLC molecules using either an external magnetic field or anexternal electric field. The resulting structure is a paired CLC-based,P₁ and P₂ information storage structure according to the presentinvention.

As discussed in great detail hereinabove, it is essential that thespiral or concentric information tracks recorded on a pair ofinformation storage layers be precisely aligned with each other in orderto be capable of simultaneously reading information from bothinformation storage layers. Thus precise alignment of paired P₁ and P₂information storage layers is required during the stamping or laminationsteps of manufacturing processes. Below are two alignment procedureswhich can be used to produce high-volume, low-cost CLC CD-ROM disks,

As shown in FIG. 27A, one method of information storage layer alignmentinvolves forming CLC alignment marks on the mold portion of eachinformation storage layer to be manufactured. The purpose of thesealignment marks is to facilitate precise alignment of the informationstorage tracks formed on the A-side and B-side of pressed informationstorage layers as they are made. Preferably, the location of thesealignment marks are written at outer rim of each stamped disk. As shownin FIG. 27B, the CLC-based alignment marks may be realized as eitherrectangular elements, circular arcs, radial lines or any other patternwhich provides positional information (x, y) or (r, θ) on the stampeddisks. The function of these CLC-based alignment marks is to producereflecting strips on both the A and B sides of the CLC disks which areboth wavelength and polarization state selective. In the illustrativeembodiment, the CLC alignment marks change a linearly polarized laserbeam into a RHCP laser beam upon reflection thereoff. The informationretrieval techniques disclosed hereinabove can be used to retrieverelative positional information from the scanned alignment marks. As thescanned area carrying the alignment marks is very small, it will be moreconvenient to use a modified optical system to retrieve relativepositional information from the scanned alignment marks. This willensure that the paired information storage layers are precisely alignedfor simultaneous information track reading operations using the opticalpick-up of FIG. 21.

In FIG. 27C, a first illustrative embodiment of the informationtrack-alignment system is shown. As shown in FIG. 27C, the opticalalignment system comprises: a laser diode (not shown) for producing alaser beam with characteristic wavelength λ_(i) and linear polarizationstate; a photodetector 2808; a collimating lens 2807; a RHCP polarizingfilter 2806; a beam splitter 2802; an oscillating plate 2803; scanningfocusing lens 2804; and focus adjustment mechanism 2805. In order toalign the information tracks on the A and B sides of a pair of stampeddiscs, the information track-alignment system operates as follows.During an alignment operation, the laser diode produces a laser beamhaving wavelength λ_(i) and polarization state P₁. The laser beam iscollimated and then directed through beamsplitter 2802, oscillatingglass plate 2803, and scanning focusing lens 2804, onto the CLC-basedalignment marks formed on the A and B sides of the paired informationstorage structure. The oscillating plate 2803 moves the light beam asindicated, resulting in a focused light spot scanning along thealignment marks formed on the A and B sides of the stamped discs of theinformation storage structure. The scanning focusing lens 2805 is movedup or down along its optical axis in order to focus on both the A and Bsides of the information storage discs. As the laser beam is reflectedoff the CLC alignment marks on the information storage layers, itspolarization state is changed to RHCP. This reflected laser beam is thenfocused by lens 2804 and directed by the beamsplitter 2802 through RHCPpolarizing filter 2806. The RHCP polarizing filter passes only the RHCPportions of the reflected laser beam corresponding to the CLC alignmentmarks. These optical signals are focused onto photodetector 2808 byfocusing lens 2807 and produce corresponding electrical signals. Thepolarizing filter is controlled by an electrical signal in synchronismwith the focusing mechanism 2804. In FIG. 27D, the produced electricalsignals are shown. The detection scheme is similar to reading the pairedinformation layers simultaneously, except that the data pattern is notrandom, as shown in FIG. 27D. Assuming the pattern is 10101 . . . , theelectrical signals are of the form of a square wave. The relative timingof the leading/trailing edges of the electrical signal providesinformation about the spatial alignment of the paired informationlayers. By timing these signals, an error signal δt can be generated foruse in servo-control during the alignment process.

In FIG. 27E, a second illustrative embodiment of the informationtrack-alignment system is shown. As shown in FIG. 27E, the opticalalignment system comprises: an image detector (i.e. CCD array) 2891; afiber-optic ring-shaped illuminator 2892 for producing a laser beam withcharacteristic wavelength λ_(i) and a linear polarization state, forillumination of the CLC alignment marks on sides A and B of theinformation recording discs; an image forming device 2893 for forming animage of an the alignment marks on the CCD detector; a mechanism 2894for moving the imaging device 2893 up or down to focus on side A or sideB of the information recording discs; a polarizing filter 2895 forpassing either a P₁ or P₂ polarized image of the alignment marks inresponse to an electrical signal synchronous to the control signalcontrolling focusing mechanism 2894. During operation, the informationtracks on discs A and B are moved slowly relative to each other, asshown in FIG. 27F, in order to achieve an aligned state. The movement ofthese discs is controlled by error signals that are produced bycomparing signals representative of alignment marks on sides A and B,produced by the image detector 2891 during alignment movement. If suchsignals have high contrast and are relatively noise-free, then it ispossible to achieve sub-micron alignment. After alignment of theinformation recording tracks has been achieved, a bonding agent inliquid form, such as UV or heat curable adhesives (e.g. cement orepoxy), is applied to the aligned discs. During alignment and beforecuring, the discs are moved relative to each. After alignment, thebonding agent is cured by UV and/or heat, and sometimes simply by thepassage of time (e.g. when induced polymerization process is used),causing the applied liquid bonding agent to solidify. After completionof this alignment process, the discs are ‘glued’ together and cannot bemoved relative to each other.

In FIG. 28, there is shown a method of manufacturing aligned CLC discsusing sheet-forming rollers and a stamping mold. As shown, partiallycured plastic is extruded to forming a plastic sheet. Then at a stampingstation, the plastic sheet is stamped using precisely aligned stampingmolds, as described in connection with FIG. 26A. The stamping molds canbe made using the techniques described hereinabove. During the stampingoperation, the disk surfaces are impressed with pit patternscorresponding to digital information recorded during the manufacturingof the stamping molds. Then the stamped disc is cured for hardness in aconventional manner. The remainder of the manufacturing steps, such asfilling the pits with liquid crystal material, capping the surfaces, andthen writing digital information values to the information storagecells, are carried out in a manner to that described above.

The above-described disc manufacturing process can be modified so thatCLC discs are made using embossing techniques rather than stampingtechniques. As shown in FIG. 29, a partially cured plastic is extrudedto forming a plastic sheet. Then at an embossing station, the plasticsheet is embossed using precisely aligned embossing drums which can bemade using techniques described in FIGS. 30A and 30B, described below.During the embossing operation, the disk surfaces are embossed with pitpatterns corresponding to digital information recorded during themanufacturing of the stamping molds. Then the embossed disc is cured forhardness in a conventional manner. The remainder of the manufacturingsteps, such as filling the pits with liquid crystal material, cappingthe surfaces, and then writing digital information values to theinformation storage cells, are carried out in a manner to that describedabove.

As shown in FIGS. 30A and 30B, the embossing drums can be made in mannerquite similar to that employed in manufacturing stamping molds. Asshown, a blank drum with a smooth surface is rotate at a prescribedangular velocity while a focused laser beam is directed at the surfacein order to photoablate the smooth surface and thus write digital datapatterns along the z-axis. The resulting surface of the embossing drumhas pit patterns of submicron depth, which correspond to the digitaldata patterns recorded therein. This technique is used to produce anembossing drum for side A and side B of the CLC disc to be manufacturedby the embossing technique. However, like the stamping manufacturingtechnique, the embossing drums must also be aligned so that theinformation tracks recorded on the resulting CLC disc are preciselyaligned so that simultaneous reading thereof may be carried using theoptical pick-up of FIG. 22 or the like. Thus, CLC alignment marks arephotoablated in the surface of the embossing drum during itsmanufacture, and thereafter filled with a CLC material, as describedabove. As shown in FIGS. 30A and 30B, the alignment marks (i.e. θ-marks)encode the angular position θ of the digital information tracks recordedin the surface of the embossing drum.

In FIG. 31, a modified procedure is shown for aligning embossing drumsused during process of FIG. 29 to simultaneously emboss side A and sideB into the opposite sides of the extruded plastic sheet. As shown, thealigning system comprises: a laser source (e.g. a He—Ne laser) forproducing a laser beam having a characteristic wavelength and linearpolarization-state; a beam-splitter BS for splitting the produced laserbeam along first and second paths; a first focusing lens disposed alongthe first path, for focusing the laser beam on the θ-marks of the firstembossing drum; a mirror M for directing the laser beam along the secondpath towards the second embossing drum; a second focusing lens disposedalong the second path, adjacent the mirror M, for focusing the laserbeam on the θ-marks of the second embossing drum; a third focusing lensdisposed along the second path as shown, for focusing the laser beamreflected off the θ-marks of the first and second embossing drums; and aphotodetector for detecting the laser beam focused by the third focusinglens. The detection scheme employed in this system is similar to thatfor aligning two discs, as shown in FIG. 27D. The major differences isthat the system shown in FIG. 31 is modified so as to use twophotodetectors, rather than one, and that polarized light having both P₁and P₂ components is used, as the alignment marks are not wavelength andpolarization selective in this system. During the drum alignmentprocedure, the first and second embossing drums are incrementallyrotated relative to each other, while the polarizing beamsplitter BS isused to divide the laser beam produced from the laser source. The P₁ andP₂ components of the produced laser beam travel along separate paths, inorder to generate reflecting signals from respective drums. Thepolarizers shown are used to isolate the reflecting signals, whereas thefocusing lenses focus the beam onto the alignment marks and focus thereflected light beams onto the photodetectors. The detection scheme issimilar to reading the paired information layers simultaneously, exceptthat the θ-marks (i.e. information pattern) is not random. Assuming theinformation pattern is 10101 . . . , the electrical signals are of theform of a square wave. The relative timing of the leading/trailing edgesof the electrical signal provides information about the spatialalignment of the embossing drums. By timing these signals, an errorsignal δt can be generated for use in servo-control during the processof precisely aligning the embossing drums.

Having described the illustrative embodiments of the present invention,several modifications readily come to mind.

For example, the optical storage media of the present invention has beendescribed in connection with the storage of digital information.However, the paired information storage structure of the presentinvention can also be used to record analog information which has beenencoded using frequency modulation (FM) encoding, which is commonly usedin the manufacture of video compact discs of either the CAV type (havinga short playing time but easy to freeze frames) or the CLV type (havinga long playing). In such an alternative embodiment, an FM encoded analogsignal is recorded in the optical storage media hereof using thefollowing procedure. As shown in FIG. 32A, a frequency-modulated (FM)analog signal is recorded on the optical media by varying the length andposition of the reflecting area along the information storage track. Incontrast with digital encoding techniques described hereinabove, therecorded signal (and thus the polarization and wavelength reflectiveselective surfaces) have a variable length and position along theinformation storage track. Then, during information reading operations,the laser beam reflected off the information pattern produces an opticalsignal, as shown in FIG. 32B, which is detected by an optical pick-uphereof to produce a corresponding electrical digital signal. Thecorresponding electrical digital signal can then be demodulated in aconventional manner to recover the underlying analog information signal,shown in FIG. 32C.

In the above-described illustrative embodiments of the presentinvention, only the “reflected” spectral components of an incident laserbeam are detected and processed during information reading operations inorder to recover recorded information patterns. In this reflection mode,there is no change in the background signal if the information storagevalue being read is a logical “0”, and there is only a change in thebackground signal level if a logical “1” (i.e. a selectively reflectiveregion along the information track) is being read. This is commonlycalled “dark-field” detection. With this technique, a weak reflectedsignal can be conditioned (e.g. amplified) without substantiallyincreasing noise and thus obtained signals with high signal to noise(SNR) ratios.

In an alternative embodiment of the present invention, shown in FIGS.17C and 17H, only “transmitted” spectral components of an incident laserbeam are detected and processed during information reading operations inorder to recover recorded information patterns. In this transmissionmode, there is no change in the background signal level if theinformation storage value being read is a logical “1” (i.e. aselectively reflective region along the information track), and there isonly a change in the background signal level if a logical “0” is beingread. This is commonly called “bright-field” detection. While thistechnique is typically less preferred than the “dark-field” technique,due to detector saturation, it may, however, in some applications, bedesirable to employ this light detector technique. The details of thisnon-retroreflective type information storage and retrieval system willbe described below.

As shown in FIG. 17C, the optical pick-up subsystem of this alternativeembodiment comprises all of the subcomponents of the optical pick-up ofFIG. 17A. Thus, as shown in FIGS. 17D and 17H, the laser beam productionunit 11701 and light detection subsystem 11706 employed in the opticalpick-up of FIG. 17C are identical to the laser beam production unit11701 and light detection subsystem 11706 employed in the opticalpick-up of FIG. 17A. What is different in the optical pick-up of FIG.17C is the addition of light collecting subsystem 11713 and lightdetection subsystem 11714 located on the side of the optical storagedisc, opposite the incident laser beam.

As shown in FIG. 17C, the light-collection subsystem 11713 comprises anultra-compact housing adapted to move in unison with housing 1709, asthe optical pick-up moves relative to the optical storage disc duringinformation reading operations. The light collection subsystem 11713also comprises two other major subcomponents, namely: a collimating lens11711 for collecting and collimating laser light beam transmittedthrough the optical storage disc; and a beam-steering means (e.g. amirror) 11712 for steering the collimated laser beam towards the lightdetection subsystem 11714, as shown. In this optical pick-upconfiguration, the focusing means 11708 requires autofocusing andtracking. Consequently, reflected laser light is detected by lightdetection subsystem 11706 and used to generate feedback signals inmanner described in connection with the optical head of FIG. 17A. As theposition of light collection subsystem 11713 relative to the informationtrack being read is less critical during information reading operations,light collection subsystem 11713 does not require autofocusing ortracking mechanisms.

The construction of light detection subsystems 11706 and 11714 issubstantially identical. The details of such subsystems can be found byreference to the description of the subsystem of FIG. 17E. However, theprimary functional difference between these two light detectionsubsystems is that the light detector of subsystem 11714 need not havemultiple image detection surfaces, typically used to implementautofocusing and tracking functions, and thus will have a simplerconstruction.

The optical information storage media of the present invention can alsobe used to directly store (i.e. record) high-resolution images forvarious micro-imaging applications. The images can represent diversetypes of information, such as text, two-level (B/W) images, or spatialmasks for use in optical processing applications, such asimage-recognition. In this alternative embodiment, the informationstorage regions of each information storage layer can be designed tofollow the form of image matrices or submatrices, organized according toa suitable information storage structure, an the first and secondinformation values recorded in each information storage layer cancorrespond to pixel values of the image to be stored.

It is also understood that with proper encoding, logical “0s can be madeto correspond to selectively reflective regions along the informationtracks of the optical media hereof, while logical “1s” are made tocorrespond to light transmissive regions therealong.

In FIGS. 15A through 15D, a method has been disclosed for fabricating apre-recorded information storage layer using vacuum-deposited chiralthin-film (e.g. vacuum-deposited inorganic oxide thin film). Asdisclosed, this particular method involves stamping a disc having aundulated surface encoded with a digital information pattern, and thendepositing a chiral coating or film over the stamped disk.

In yet a further alternative embodiment of the present invention,illustrated in FIGS. 33F and 33G, a paired information storage structureof the type illustrated in FIGS. 9E, 9F 9G and 9H, can be made using apair of patterned layers of artificial chiral thin-film havingcharacteristic wavelengths and polarization states (λ_(i), P₁) and(λ_(i), P₂). Such a pair of information storage structure can be used toform multi-layered and multi-decked information structures in accordancewith the principles disclosed herein.

In FIGS. 33A through 33G, a method is presented for forming amulti-layered information storage device, in which digital informationvalue storage locations of either the first or second binary value arerealized as a patterned layer of artificial chiral thin-film having aparticular characteristic wavelength and polarization state, namely(λ_(i), P₁) or (λ_(i), P₂). In FIGS. 33A through 33G, steps areillustrated for forming the i-th information storage layer in amulti-layered information storage structure. As illustrated in FIG. 33A,the first step of the method involves depositing upon the i-th substrateenclosed within a vacuum environment, a uniform layer of artificialchiral film which exhibits wavelength and polarization selectivereflectance at a characteristic wavelength λ_(i) and polarization stateP₂. A suitable technique for depositing chiral film has been describedabove, with reference to Azzam, supra. Suitable material for depositingbirefringent film and chiral film (i.e. a rotating birefringent film)include oxides of two groups of elements selected from the PeriodicTable for Elements: the first group on the left side of the Tableincludes the elements Li, Ba, Y, Ti, Zr, Hf, V, Nb, Ta, Mo, W, and Re;and second group on the right side of the Table includes B, Si, Ge, Sn,Pb, Sb, Bi. Such materials are identified in the 1989 paper “Thin filmretardation plate by Oblique deposition” by T. Motohiro and Y. Taga,published in Applied Optics, Vol. 28, No. 13, at page 2466.

As shown in FIG. 33B, the next step of the method involves applying alayer of photoresist to the chiral film layer, and after exposing thephotoresist layer to a pattern of actinic light formed by a photo-maskinterposed between the photoresist layer and the source of the light.The light pattern formed in the photo-mask spatially corresponds toeither the first digital information value pattern (e.g. a logical “1”pattern) that is to be formed on the (λ_(i), P₂) information storagelayer. The logical complement of the light pattern spatially correspondsto the second digital information value pattern (e.g. a logical “0”pattern) that is to be formed on the (λ_(i), P₂) information storagelayer. After removing the photo-mask and using photo-resist etchantsknown in the art to etch away the photo-resist layer at locations notexposed to light through the photo-mask, a photo-resist pattern isformed on the chiral film layer, as shown in FIG. 33C. This photo-resistpattern spatially corresponds to the first digital information valuepattern (e.g. a logical “1” pattern) associated with the digitalinformation pattern to be formed on the (λ_(i), P₂) information storagelayer. Then the portions of the chiral layer unprotected by theremaining photo-resist pattern are etched away using chemical etchantsknown in the art, to form a chiral thin-film pattern which spatiallycorresponds to the first digital information value (i.e. bit) pattern(e.g. a logical “1” pattern) that is to be formed on the (λ_(i), P₂)information storage layer.

As shown in FIG. 33E, the next step of the method involves applying anoptically transparent filler material to the resulting chiral thin-filmpattern in order to fill in the logical complement (e.g. logical “0”pattern) presented upon the i-th substrate, and thus achieveplanarization. This filler material should be substantially transparentto the optical wavelengths over which the information storage device isdesigned to operate. Preferably, the filler material has an index ofrefraction approximately equal to that of the average refractive indexvalue of the chiral film. The advantage of using filler material withsuch properties is that, when reading information recorded in lowerinformation storage layers, the laser reading beam sees an approximatelyuniform material, thus resulting in lower scattering losses in theinformation-encoded return signal.

Notably, the resulting structure fabricated by the above-describedprocess provides only one half of the i-th paired information storagestructure. At this stage, there are two ways in which to proceed informing the i-th paired information storage structure according to theprinciples of the present invention.

The first approach involves repeating the above steps described at FIGS.33A through 33E, using an uniform layer of artificial chiral film havinga characteristic wavelength λ_(i) and polarization state P₁, instead ofa uniform layer of artificial chiral film having a characteristicwavelength λ_(i) and polarization state P₂, where polarization state P₂is orthogonal to polarization state P₁. Then as shown in FIG. 33F, thesetwo information storage structures are laminated together in a manner sothat the chiral thin-film pattern associated with the (λ_(i), P₁)information storage layer is adjacent the chiral thin-film patternassociated with the (λ_(i), P₂) information storage layer. In thisapproach, the chiral-based information storage structures are interposedbetween the i-th and (i−1)-th substrates within the multilayeredinformation storage structure.

The second approach also involves repeating the above steps described atFIGS. 33A through 33E, using an uniform layer of artificial chiral filmhaving a characteristic wavelength λ_(i) and polarization state P₁,instead of an uniform layer of artificial chiral film having acharacteristic wavelength λ_(i) and polarization state P₂. Then as shownin FIG. 33G, these two information storage structures are laminatedtogether in a manner so that the chiral thin-film pattern associatedwith the (λ_(i), P₂) information storage layer is separated from thechiral thin-film pattern associated with the (λ_(i), P₁) informationstorage layer by the (i−1)-th substrate within the multilayeredinformation storage structure.

Having fabricated the i-th information storage layer within themulti-layered structure, the above-described process is repeated foreach (i+1)-th paired information storage structure. Each adjacent layercan be laminated to its neighboring layer to provide the multilayeredinformation storage structure having light reflection properties thatare response to both the wavelength and polarization state of theincident light beam used to read information values prerecorded therein.Multilayered information storage structures, in turn, can be assembledtogether to form a multi-decked information storage device, as describedhereinabove.

The above-described method of making information storage structures ofthe present invention from chiral thin-film can be readily applied tomake information storage structures of the present invention usingdielectric birefringent film, as shown in FIG. 16. The majormodification is that chiral material is replaced with dielectricbirefringent material. Also, birefringent film employed in the structureof FIG. 16 can also be fabricated by vacuum depositing multilayer filmon an oblique substrate. In this case, the substrate is not rotating butis fixed in two orientations, namely: normal in XZ and YZ planes.

The modifications described above are merely exemplary. It is understoodthat other modifications to the illustrative embodiments will readilyoccur to persons with ordinary skill in the art. All such modificationsand variations are deemed to be within the scope and spirit of thepresent invention as defined by the accompanying claims to Invention.

What is claimed is:
 1. An information storage device comprising: a plurality of information storage structures, each said i-th information storage structure being indexable as an i-th information storage structure where i=1, 2, 3, . . . N and each said i-th information storage structure having an undulated surface with first and second surface heights and a plurality of first and second information value storage locations recorded in each said i-th information storage structure and corresponding to said first and second surface heights, respectively; and wherein each said first and second information value storage location recorded in said undulated surface of said i-th information structure (i) is characterized by a characteristic wavelength band Δλ_(i) defined about a prespecified wavelength λ_(i), (ii) reflects spectral components that are incident to said first or second information value storage location, and have a wavelength within said characteristic wavelength band Δλ_(i), and (iii) transmits spectral components that are incident to said first or second information value storage location and have a wavelength outside of said characteristic wavelength band Δλ_(i), so that an optical pick-up, arranged for detecting spectral components incident to said i-th information storage structure, is capable of detecting at a first signal level, spectral components reflected from each said first information value storage location in said i-th information storage structure, and detecting at a second signal level different than said first signal level, spectral components reflected from each said second information value storage location in said i-th information storage structure.
 2. The information storage device of claim 1, wherein each said first information value is a logical ‘1’ and each said second information value is a logical ‘0’.
 3. The information storage device of claim 1, wherein said undulated surface in each said i-th information storage structure is made from an artificial chiral film.
 4. The information storage device of claim 1, wherein said undulated surface in each said i-th information storage structure is made from a plurality of dielectric layers.
 5. An information storage and retrieval system for reading information values recorded in an information storage device constructed from an optical based storage media, said information storage and retrieval system comprising: light beam producing means for producing a light beam including at least one spectral component having a wavelength within a characteristic wavelength band Δλ_(i) defined about a prespecified wavelength λ_(i); supporting means for supporting, during an information reading operation, an information storage including a plurality of information storage structures, each said information storage structure being indexable as an i-th information storage structure where i=1, 2, 3, . . . N and each said information storage structure having an undulated surface with first and second surface heights and a plurality of first and second information value storage locations recorded in said i-th information storage structure and corresponding to said first and second surface heights, respectively, and wherein each said first and second information value storage location recorded in said undulated surface of said i-th information storage structure (i) is characterized by a characteristic wavelength band Δλ_(i), (ii) reflects spectral components that are incident to said first or second information storage structure, and have a wavelength within said characteristic wavelength band Δλ_(i), and (iii) transmits spectral components that are incident to said first or second information value storage structure and have a wavelength outside of said characteristic wavelength band Δλ_(i), light focusing means for focusing said light beam during the reading of information values from one of said first and second information storage structures; light detecting means arranged for detecting spectral components incident to said i-th information storage structure, and detecting at a first signal level, spectral components reflected from each said first information value storage location in said i-th information storage structure, and detecting at a second signal level different than said first signal level, spectral components reflected from each said second information value storage location in said i-th information storage structure; spectral component analyzing means for analyzing the detected signal level of the spectral components detected by said light detection means, determining the information values recorded in said i-th information storage structure and producing a data stream representative of said determined information values; and information access control means for controlling the operation of said light beam producing means during information reading operation.
 6. The information storage and retrieval system of claim 5, which further comprises moving means for moving said information storage device relative to said focused laser beam so that, during the reading of information from said i-th information storage structure, at least one spectral component of said incident light beam is reflected from one or more of said first plurality of first and second information value storage locations recorded in said i-th information storage structure.
 7. The information storage and retrieval system of claim 5, wherein said light beam is a laser beam.
 8. The information storage and retrieval system of claim 5, which further comprises: interface means for interfacing said information storage and retrieval system with a host computer system.
 9. The information storage and retrieval system of claim 5, which further comprises address translation means for translating a logical address assigned to each said information value storage location in said i-th information storage structure, into a physical address specifying the physical location of said information value storage location in said information storage device.
 10. The information storage and retrieval system of claim 5, wherein each said first information value storage location stores a logical ‘1’ and each said second information value storage location stores a logical ‘0’.
 11. An information storage and retrieval system for reading information values recorded in an information storage device constructed from multiple layers of optical storage media, said information storage and retrieval system comprising: light beam producing means for producing a light beam including a first spectral component having a prespecified wavelength λ_(i) and a first characteristic polarization state P1, and a second spectral component having said prespecified wavelength λ_(i) and a second characteristic polarization state P2 orthogonal to said first characteristic polarization state P1; spectral component selection means for selecting the production of one of (i) said first spectral component and (ii) said second spectral component, during an information reading operation; supporting means for supporting, during said information reading operation, an information storage device constructed from a plurality of information storage layers of optical storage media each said i-th information storage layer being indexable as an i-th information storage layer where i=1, 2, 3, . . . N, and collectively being stacked together to form a composite structure, wherein each said i-th information storage layer includes a first information storage structure disposed adjacent a second information storage structure, said first information storage structure of each said i-th information storage layer having a first undulated surface and a first plurality of first and second information value storage locations recorded in said first undulated surface, wherein each said first and second information value storage location recorded in said first undulated surface of said i-th information storage layer is characterized by a first characteristic wavelength band Δλ_(a) and said first characteristic polarization state P₁, reflects spectral components that are incident to said i-th information storage layer and have a wavelength within said first characteristic wavelength band Δλ_(a) and a polarization state equal to said first characteristic polarization state P₁, and transmits spectral components that are incident to said i-th information storage layer and have at least one of (i) a wavelength outside of said first characteristic wavelength band Δλ_(a) and (ii) a polarization state which is equal to said second characteristic polarization state P₂, and said second information storage structure of each of said i-th information storage layer having a second undulated surface and second plurality of first and second information value storage locations recorded in said undulated surface, wherein each said first and second information value storage location recorded in said second undulated surface of said i-th information storage layer is characterized by said characteristic polarization state P₂ and a second characteristic wavelength band Δλ_(b) overlapping at least a portion of said first characteristic wavelength band Δλ_(a) located around said prespecified wavelength λ_(i), reflects spectral components that are incident to said i-th information storage layer and have a wavelength within said second characteristic wavelength band Δλ_(b) and a polarization state equal to said second characteristic polarization state P₂, and transmits spectral components that are incident to said i-th information storage layer and have at least one of (i) a wavelength outside of said second characteristic wavelength band Δλ_(b) and (ii) a polarization state equal to said first characteristic polarization state P₁; light focusing means for focusing said light beam while reading said first and second information value storage locations from said first and second information storage structures of said i-th storage layer; light detecting means arranged for detecting spectral components incident to said i-th information storage layer, and detecting at a first signal level, spectral components reflected from each said first information value storage location in said i-th information storage layer, and detecting at a second signal level different than said second signal level, spectral components reflected from each said second information value storage location in said i-th information storage layer; spectral component analyzing means for analyzing the signal level of the spectral components detected by said light detection means, determining the information values recorded in the first information storage structure of said i-th information storage layer and producing a first data stream representative of said first spectral component reflected from said first information storage structure, and determining the information values recorded in the second information storage structure of said i-th information storage layer and producing a second data stream representative of said second spectral component reflected from said second information storage structure; and information access control means for controlling the operation of said light beam producing means during said information reading operation.
 12. The information storage and retrieval system of claim 11, which further comprises moving means for moving said information storage device relative to said focused laser beam so that, during the reading of information from said i-th information storage layer, the first spectral component of said focused laser beam is reflected from at least one of said first plurality of first and second information value storage locations recorded in said i-th information storage layer, and during the reading of information from said i-th information storage layer, the second spectral component of said focused laser beam is reflected from at least one of said second plurality of said information value storage locations recorded in said i-th information storage layer.
 13. The information storage and retrieval system of claim 12, wherein said light beam is a laser beam.
 14. The information storage and retrieval system of claim 12, which further comprises: interface means for interfacing said information storage and retrieval system with a host computer system.
 15. The information storage and retrieval system of claim 12, which further comprises address translation means for translating a logical address assigned to each said information value storage location in said first and second information storage structures, into a physical address specifying the physical location of said information value storage location in said information storage device.
 16. The information storage and retrieval system of claim 12, wherein each said first information value storage location store a logical ‘1’ and each said second information value storage location stores a logical ‘0’. 